best time to buy and sell stocks III

对于这题，题目要求应该是最多交易两次

对于同一天内买进又卖出相当与没有参与交易

所以对于给出的一列数组【a1,a2,a3,...ai.....an】

可以分为两段在ai处，【a1....ai】的总利润+【a(i+1),...an】的利润=利润

将数组变成差分数组，分别求两段对应的利润

 class Solution {public:    int maxProfit(vector<int>& prices) {        if (prices.size() < 2) return 0;        const int n = prices.size();        vector<int> f(n, 0);        vector<int> g(n, 0);        for (int i = 1, valley = prices[0]; i < n; ++i) {            valley = min(valley, prices[i]);            f[i] = max(f[i - 1], prices[i] - valley);        }        for (int i = n - 2, peak = prices[n - 1]; i >= 0; --i) {            peak = max(peak, prices[i]);            g[i] = max(g[i], peak - prices[i]);        }        int max_profit = 0;        for (int i = 0; i < n; ++i)            max_profit = max(max_profit, f[i] + g[i]);        return max_profit;    }};