[LeetCode] Best Time to Buy and Sell Stock

Say you have an array for which the i^th element is the price of a given stock on day i.

If you were only permitted to complete at most one transaction (ie, buy one and sell one share of the stock), design an algorithm to find the maximum profit.

用類似動態規划的思想，到第i天買入，那么我能賺到的最大利潤是多少呢？就是i + 1 ~ n天中最大的股價減去第i天的。找最大股價的問題可以在找第i+1~n天的最大利潤時順便記錄，

這樣就得出了一個線性方法。

class Solution {
public:
    int maxProfit(vector<int> &prices) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (prices.size() == 0)
            return 0;
            
        int maxPrice = prices[prices.size()-1];
        int ans = 0;
        for(int i = prices.size() - 1; i >= 0; i--)
        {
            maxPrice = max(maxPrice, prices[i]);
            ans = max(ans, maxPrice - prices[i]);
        }
        
        return ans;
    }
};