【LeetCode】Best Time to Buy and Sell Stock

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.

这题是《数据结构与算法分析：C语言描述》里举得例子，最大子数组和。我是先计算相邻元素的差，保存到数组diff里，然后计算数组diff的最大子数组和，其实没必要这样，不过无所谓了。

class Solution {
public:
    int maxProfit(vector<int> &prices) {
        if (prices.empty()) {
            return 0;
        }
        
        vector<int> diff;
        for (int i = 1; i < prices.size(); ++i) {
            diff.push_back(prices[i] - prices[i-1]);
        }
        
        return maxSubarray(diff);
    }
    
    int maxSubarray(vector<int> &array) {
        int sum  = 0, max_sum = 0;
        for (int i = 0; i < array.size(); ++i) {
            sum += array[i];
            if (max_sum < sum) {
                max_sum = sum;
            } else if (sum < 0) {
                sum = 0;
            }
        }
        
        return max_sum;
    }
};