Solution for permutation related questions

Author: andy2git

Next_Permutation.cpp

@@ -0,0 +1,31 @@
+class Solution {
+public:
+    void nextPermutation(vector<int> &num) {
+        
+        int k = find_break(num);
+        
+        if(k == -1){
+            sort(num.begin(), num.end());
+        }else{
+            sort(num.begin()+k+1, num.end());
+            
+            for(int i = k+1; i < num.size(); i++){
+                if(num[i] > num[k]){
+                    swap(num[k], num[i]);
+                    break;
+                }
+            }
+        }
+    }
+
+private:
+    int find_break(vector<int> &A){
+        if(A.size() < 2) return -1;
+        
+        for(int i = A.size()-1; i>0; i--){
+            if(A[i] > A[i-1]) return i-1;
+        }
+        
+        return -1;
+    }
+};

Permutation_Sequence.cpp

@@ -0,0 +1,44 @@
+class Solution {
+public:
+    string getPermutation(int n, int k) {
+        vector<int> elems;
+        for(int i = 1; i <= n; i++){
+            elems.push_back(i);
+        }
+        
+        return perm_helper(elems, k);
+    }
+private:
+    int fac(int i){
+        if(i == 1) return 1;
+        int result = 1;
+        while(i > 1){
+            result *= i;
+            i--;
+        }
+        return result;
+    }
+    
+    string perm_helper(vector<int> &A, int k){
+        string result;
+        
+        if(k == 1){
+            for(int i = 0; i < A.size(); i++){
+                result += (char)(A[i] + '0');
+            }
+            return result;
+        }
+        
+        int range = fac(A.size()-1);
+        int ind = (k-1)/range;
+        
+        int new_rank = k%range;
+        if(new_rank == 0) new_rank += range;
+        
+        result += (char)(A[ind]+'0');
+        A.erase(A.begin()+ind);
+        
+        result += perm_helper(A, new_rank);
+        return result;
+    }
+};

Permutations.cpp

@@ -0,0 +1,30 @@
+class Solution {
+public:
+    vector<vector<int> > permute(vector<int> &num) {
+        if(num.empty()) return vector<vector<int>>();
+        vector<vector<int>> result;
+        
+        vector<int> sofar;
+        vector<int> rest;
+        permute_helper(result, sofar, num);
+        
+        return result;
+    }
+    
+private:
+    void permute_helper(vector<vector<int>> &result, vector<int> &sofar, vector<int> &rest){
+        if(rest.empty()){
+            result.push_back(sofar);
+            return;
+        }
+        
+        for(int i = 0; i < rest.size(); i++){
+            int t = rest[i];
+            sofar.push_back(t);
+            auto it = rest.erase(rest.begin()+i);
+            permute_helper(result, sofar, rest);
+            sofar.pop_back();
+            rest.insert(it, t);
+        }
+    }
+};

Permutations_2.cpp

@@ -0,0 +1,33 @@
+class Solution {
+public:
+    vector<vector<int> > permuteUnique(vector<int> &num) {
+        if(num.empty()) return vector<vector<int>>();
+        sort(num.begin(), num.end());
+        
+        vector<vector<int>> result;
+        vector<int> sofar;
+        permute_helper(result, sofar, num);
+        
+        return result;
+    }
+private:
+    void permute_helper(vector<vector<int>> &result, vector<int> &sofar, vector<int> &rest){
+        if(rest.empty()){
+            result.push_back(sofar);
+            return;
+        }
+        
+        for(int i = 0; i < rest.size(); i++){
+            if(i > 0 && rest[i] == rest[i-1]) continue;
+            int t = rest[i];
+            
+            sofar.push_back(t);
+            auto it = rest.erase(rest.begin()+i);
+            
+            permute_helper(result, sofar, rest);
+            
+            sofar.pop_back();
+            rest.insert(it, t);
+        }
+    }
+};