121、Best Time to Buy and Sell Stock

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

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

Note that you cannot sell a stock before you buy one.

Example 1:

Input: [7,1,5,3,6,4]
Output: 5
Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 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
Explanation: In this case, no transaction is done, i.e. max profit = 0.

思路

遍历一遍，记录买入的最小值，每次计算当前价格卖出价格和最小值买入的利润，记录当前利润，并和后续进行比较，得到最大利润。

c++11

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int max_profit = 0, buy_price = INT_MAX;
        for (int cur_price : prices) {
            buy_price = std::min(buy_price, cur_price);
            max_profit = std::max(max_profit, cur_price - buy_price);
        }
        return max_profit;
    }
};

AC结果：

Runtime: 8 ms, faster than 99.61% of C++ online submissions for Best Time to Buy and Sell Stock.
Memory Usage: 9.4 MB, less than 81.66% of C++ online submissions for Best Time to Buy and Sell Stock.