Merge pull request #6 from colorbox/121

Add solved codes for `121. Best Time to Buy and Sell Stock`

Author: colorbox

121/1.cpp

@@ -0,0 +1,26 @@
+class Solution {
+public:
+    int maxProfit(vector<int>& prices) {
+        vector<int> minPricesFromStart;
+        vector<int> maxPricesFromEnd;
+
+        int minPrice = INT_MAX;
+        for(int i = 0; i < prices.size(); i++){
+            minPrice = min(minPrice, prices[i]);
+            minPricesFromStart.push_back(minPrice);
+        }
+        int maxPrice = -1;
+        for(int i = prices.size()-1; i >= 0; i--){
+            maxPrice = max(maxPrice, prices[i]);
+            maxPricesFromEnd.push_back(maxPrice);
+        }
+        reverse(maxPricesFromEnd.begin(), maxPricesFromEnd.end());
+
+        int maxProfit = 0;
+        for(int i = 0; i < prices.size(); i++){
+            int currentProfit = maxPricesFromEnd[i] - minPricesFromStart[i];
+            maxProfit = max(maxProfit, currentProfit);
+        }
+        return maxProfit;
+    }
+};

121/2.cpp

@@ -0,0 +1,18 @@
+class Solution {
+public:
+    int maxProfit(vector<int>& prices) {
+        int maxPrice = 0;
+        int minPrice = INT_MAX;
+        int maxProfit = 0;
+        for (int i = 0; i < prices.size(); i++) {
+            if (prices[i] < minPrice) {
+                minPrice = prices[i];
+            } else {
+                maxPrice = prices[i];
+                int currentProfit = maxPrice - minPrice;
+                maxProfit = max(maxProfit, currentProfit);
+            }
+        }
+        return maxProfit;
+    }
+};

121/3.cpp

@@ -0,0 +1,16 @@
+class Solution {
+public:
+    int maxProfit(vector<int>& prices) {
+        int minPrice = INT_MAX;
+        int maxProfit = 0;
+        for(int i = 0; i < prices.size(); i++) {
+            if (prices[i] < minPrice) {
+                minPrice = prices[i];
+            } else {
+                maxProfit = max(maxProfit, prices[i] - minPrice);
+            }
+        }
+
+        return maxProfit;
+    }
+};

121/4.cpp

@@ -0,0 +1,18 @@
+class Solution {
+public:
+    int maxProfit(vector<int>& prices) {
+        if(prices.empty())abort();
+        
+        int minPrice = prices[0];
+        int currentProfit = 0;
+        for(int price: prices) {
+            if (price < minPrice) {
+                minPrice = price;
+            } else {
+                currentProfit = max(currentProfit, price - minPrice);
+            }
+        }
+
+        return currentProfit;
+    }
+};