LeetCode：Best Time to Buy and Sell Stock

描述
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.
分析
解法一：使用貪心策略，分別找到價格最低和最高的一天，低進高出，注意最低的一天要在最高的一天之前。時間複雜度爲O（n）, 空間複雜度爲O（1）.
代碼如下

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

解法二：把原始價格序列變成差分序列，本題也可以看做是最大m子段和，m=1。