89：Best Time to Buy and Sell Stock

题目：Say you have an array for which the i-th 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.

解题代码如下：

// 时间复杂度 O(n)，空间复杂度 O(1)
class Solution {
public:
        int maxProfit(vector<int>& prices) {
                if (prices.size() < 2) return 0;
                int profit = 0; // 表示利润
                int cur_min = prices[0];

                // 将第 i + 1 天的价格减去前 i 天中最低的价格
                // 前 i 天中最低的价格用 cur_min 表示
                for (int i = 1; i < prices.size(); ++i) {
                        profit = max(profit, prices[i] - cur_min);
                        cur_min = min(cur_min, prices[i]);
                }
                return profit;
        }