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.

 

思路：从后往前扫，维护后面的最大值。或者从前往后扫，维护前面的最小值

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int n = prices.size();
        int Max = 0;
        int ans = 0;
        for (int i = n - 1; i >= 0; --i) {
            Max = max(prices[i], Max);
            ans = max(Max - prices[i],ans);
        }
        return ans;
    }
};