121. Best Time to Buy and Sell Stock

This is actually a maximum subarray sum problem which can be solved by Kadane’s Algorithm.
Here is a very good video explaining this algorithm:
https://www.youtube.com/watch?v=86CQq3pKSUw

1:  class Solution {  
2:  public:  
3:    int maxProfit(vector<int>& prices) {  
4:      int maxGlobal = 0;  
5:      int maxCurrent = 0;  
6:      for (int i = 1; i < prices.size(); i++) {  
7:        maxCurrent = max(prices[i]-prices[i-1], maxCurrent + prices[i]-prices[i-1]);  
8:        if (maxCurrent > maxGlobal) maxGlobal = maxCurrent;  
9:      }  
10:      return maxGlobal;  
11:    }  
12:  };