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24 medium problems
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‎3sum-closest_1_AC.cpp

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#include <algorithm>
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#include <cmath>
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using std::abs;
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using std::sort;
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class Solution {
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public:
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int threeSumClosest(vector<int>& nums, int target) {
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vector<int> &a = nums;
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int n = a.size();
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if (n < 3) {
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return 0;
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}
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sort(a.begin(), a.end());
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int res = a[0] + a[1] + a[2];
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int i, j, k;
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int val;
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for (i = 0; i < n - 2; ++i) {
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j = i + 1;
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k = n - 1;
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while (j < k) {
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val = a[i] + a[j] + a[k];
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if (abs(val - target) < abs(res - target)) {
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res = val;
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}
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if (val < target) {
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++j;
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} else if (val > target) {
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--k;
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} else {
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return target;
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}
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}
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}
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return res;
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}
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};

‎basic-calculator-ii_1_AC.cpp

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// So there's a way to hash put lambda function in a hash table after all.
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// That's a relief.
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#include <functional>
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#include <unordered_map>
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using std::function;
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using std::unordered_map;
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class Solution {
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public:
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Solution() {
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fun["+"] = [](int x, int y) {return x + y;};
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fun["-"] = [](int x, int y) {return x - y;};
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fun["*"] = [](int x, int y) {return x * y;};
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fun["/"] = [](int x, int y) {return x / y;};
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pre["("] = -1;
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int p = 0;
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pre["+"] = p;
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pre["-"] = p;
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++p;
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pre["*"] = p;
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pre["/"] = p;
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++p;
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}
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int calculate(string s) {
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int ls = s.size();
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int res = 0;
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stack<int> nums;
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stack<string> ops;
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int i = 0;
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string op;
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while (i < ls) {
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if (s[i] == '+' || s[i] == '-' || s[i] == '*' ||
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s[i] == '/') {
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op.clear();
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op.push_back(s[i++]);
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while (!ops.empty() && pre[ops.top()] >= pre[op]) {
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calc(nums, ops);
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}
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ops.push(op);
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} else if (s[i] == '(') {
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op.clear();
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op.push_back(s[i++]);
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ops.push(op);
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} else if (s[i] == ')') {
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while (ops.top() != "(") {
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calc(nums, ops);
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}
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ops.pop();
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++i;
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} else if (isdigit(s[i])) {
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int val = 0;
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while (i < ls && isdigit(s[i])) {
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val = val * 10 + (s[i] - '0');
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++i;
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}
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nums.push(val);
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} else {
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++i;
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}
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}
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while (!ops.empty()) {
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calc(nums, ops);
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}
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res = nums.top();
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nums.pop();
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return res;
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}
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void calc(stack<int> &nums, stack<string> &ops) {
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if (nums.size() < 2 || ops.size() < 1) {
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return;
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}
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string op = ops.top();
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ops.pop();
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int n2 = nums.top();
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nums.pop();
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int n1 = nums.top();
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nums.pop();
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nums.push(fun[op](n1, n2));
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}
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~Solution() {
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fun.clear();
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pre.clear();
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}
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private:
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unordered_map<string, function<int(int, int)>> fun;
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unordered_map<string, int> pre;
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};

‎basic-calculator_1_AC.cpp

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#include <functional>
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#include <unordered_map>
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using std::function;
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using std::unordered_map;
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class Solution {
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public:
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Solution() {
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fun["+"] = [](int x, int y) {return x + y;};
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fun["-"] = [](int x, int y) {return x - y;};
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fun["*"] = [](int x, int y) {return x * y;};
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fun["/"] = [](int x, int y) {return x / y;};
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pre["("] = -1;
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int p = 0;
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pre["+"] = p;
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pre["-"] = p;
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++p;
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pre["*"] = p;
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pre["/"] = p;
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++p;
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}
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int calculate(string s) {
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int ls = s.size();
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int res = 0;
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stack<int> nums;
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stack<string> ops;
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int i = 0;
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string op;
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while (i < ls) {
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if (s[i] == '+' || s[i] == '-' || s[i] == '*' ||
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s[i] == '/') {
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op.clear();
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op.push_back(s[i++]);
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while (!ops.empty() && pre[ops.top()] >= pre[op]) {
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calc(nums, ops);
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}
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ops.push(op);
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} else if (s[i] == '(') {
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op.clear();
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op.push_back(s[i++]);
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ops.push(op);
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} else if (s[i] == ')') {
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while (ops.top() != "(") {
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calc(nums, ops);
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}
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ops.pop();
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++i;
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} else if (isdigit(s[i])) {
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int val = 0;
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while (i < ls && isdigit(s[i])) {
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val = val * 10 + (s[i] - '0');
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++i;
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}
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nums.push(val);
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} else {
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++i;
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}
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}
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while (!ops.empty()) {
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calc(nums, ops);
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}
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res = nums.top();
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nums.pop();
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return res;
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}
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void calc(stack<int> &nums, stack<string> &ops) {
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if (nums.size() < 2 || ops.size() < 1) {
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return;
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}
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string op = ops.top();
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ops.pop();
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int n2 = nums.top();
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nums.pop();
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int n1 = nums.top();
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nums.pop();
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nums.push(fun[op](n1, n2));
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}
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~Solution() {
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fun.clear();
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pre.clear();
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}
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private:
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unordered_map<string, function<int(int, int)>> fun;
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unordered_map<string, int> pre;
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};

‎course-schedule_1_AC.cpp

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// Topological sort using BFS.
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#include <queue>
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#include <vector>
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using std::queue;
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using std::vector;
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class Solution {
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public:
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bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
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int nv = numCourses;
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vector<vector<int>> g(nv);
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vector<int> ind(nv, 0);
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auto &e = prerequisites;
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queue<int> q;
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int x, y;
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int ne = e.size();
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int i;
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for (i = 0; i < ne; ++i) {
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x = e[i].second;
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y = e[i].first;
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g[x].push_back(y);
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++ind[y];
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}
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for (i = 0; i < nv; ++i) {
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if (ind[i] == 0) {
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q.push(i);
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}
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}
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int ec = ne;
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while (!q.empty()) {
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x = q.front();
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q.pop();
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while (g[x].size() > 0) {
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y = g[x].back();
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g[x].pop_back();
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--ind[y];
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--ec;
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if (ind[y] == 0) {
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q.push(y);
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}
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}
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}
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return ec == 0;
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}
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};

‎find-k-pairs-with-smallest-sums_1_AC.cpp

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#include <queue>
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using std::priority_queue;
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typedef struct Pair {
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vector<int> *a1, *a2;
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int i1, i2;
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bool operator < (const Pair &p) const {
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return (*a1)[i1] + (*a2)[i2] < (*p.a1)[p.i1] + (*p.a2)[p.i2];
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}
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bool operator > (const Pair &p) const {
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return (*a1)[i1] + (*a2)[i2] > (*p.a1)[p.i1] + (*p.a2)[p.i2];
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}
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bool operator == (const Pair &p) const {
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return (*a1)[i1] + (*a2)[i2] == (*p.a1)[p.i1] + (*p.a2)[p.i2];
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}
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} Pair;
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class Solution {
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public:
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vector<pair<int, int>> kSmallestPairs(vector<int>& nums1, vector<int>& nums2, int k) {
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vector<pair<int, int>> res;
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vector<int> &a1 = nums1;
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vector<int> &a2 = nums2;
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int n1 = a1.size();
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int n2 = a2.size();
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if (n1 == 0 || n2 == 0) {
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return res;
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}
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priority_queue<Pair, vector<Pair>, greater<Pair>> pq;
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int i;
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Pair p;
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p.a1 = &a1;
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p.a2 = &a2;
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for (i = 0; i < n1; ++i) {
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p.i1 = i;
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p.i2 = 0;
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pq.push(p);
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}
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for (i = 0; i < k && !pq.empty(); ++i) {
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p = pq.top();
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pq.pop();
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res.push_back(make_pair((*p.a1)[p.i1], (*p.a2)[p.i2]));
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++p.i2;
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if (p.i2 < (*p.a2).size()) {
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pq.push(p);
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}
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}
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while (!pq.empty()) {
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pq.pop();
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}
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return res;
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}
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};

‎gas-station_1_AC.cpp

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class Solution {
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public:
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int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
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int n = gas.size();
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if (n == 0) {
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return true;
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}
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int sum = 0;
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int i, j;
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j = 0;
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for (i = 0; i < 2 * n - 1; ++i) {
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if (i - j == n) {
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break;
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}
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sum += gas[i % n] - cost[i % n];
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if (sum < 0) {
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j = i + 1;
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sum = 0;
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}
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}
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return i - j == n ? j : -1;
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}
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};

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