Leetcode:121. Best Time to Buy and Sell Stock

Description:

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.
本题关键是要先买后卖，而且买的价格一定要比卖的少，否则输出为0

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