Create 121. Best Time to Buy and Sell Stock.md

121. Best Time to Buy and Sell Stock.md

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+# [121. Best Time to Buy and Sell Stock](https://leetcode.com/problems/best-time-to-buy-and-sell-stock/description/)
+# 思路
+题目的意思就是求数组prices中，prices[i]-prices[j]的最大值,其中i > j。  
+因此prices[j]肯定是prices[i]之前的元素中最小的那一个，所以从前往后遍历，并用min_price记录当前位置前的元素中最小的一个。  
+另外注意单独判断空数组的情况。
+# C++
+```
+class Solution {
+public:
+    int maxProfit(vector<int>& prices) {
+        if(prices.size() == 0) return 0;
+        int max_profit = 0, min_price = prices[0];
+        for(int i = 1; i < prices.size(); i++){
+            if(prices[i] - min_price > max_profit) max_profit = prices[i] - min_price;
+            if(min_price > prices[i]) min_price = prices[i];
+        }
+        return max_profit;
+    }
+};
+```