[LeetCode] Best Time to Buy and Sell Stock

题目：

Say you have an array for which the ith 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.

解答：

class Solution {public:    int maxProfit(vector<int> &prices) {        int n = prices.size();        int max_profit = 0;        int buy_index = 0;                for(int i = 0; i < n; i++) {            if(prices[i] - prices[buy_index] > max_profit) {                max_profit = prices[i] - prices[buy_index];            }            if(prices[i] < prices[buy_index]) {                buy_index = i;            }        }                return max_profit;    }};

思路：在遍历的过程中需要做两件事：（1）判断当前股价是否为历史最低，若是，更新最低股价index；（2）当前股价减去历史最低股价是否为最大收益，若是，更新最大收益。