第五周：[Leetcode]121. Best Time to Buy and Sell Stock

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.
Example 1:
Input: [7, 1, 5, 3, 6, 4]
Output: 5
max. difference = 6-1 = 5 (not 7-1 = 6, as selling price needs to be larger than buying price)
Example 2:
Input: [7, 6, 4, 3, 1]
Output: 0
In this case, no transaction is done, i.e. max profit = 0.

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寻找数组中最大值与最小值之差，最小值的下标应该在最大值之前。使用一个变量维护当前数组的最小值，向后遍历，寻找最大的差值。

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class Solution {public:    int maxProfit(vector<int>& prices) {        if(prices.size() < 2)            return 0;        int ans = 0,m = prices[0],i,profit;        for(i = 1;i < prices.size();i++){            profit = prices[i] - m;            ans = max(profit,ans);            m = min(prices[i],m);        }    return ans;    }};