【动态规划】买卖股票的最佳时机Ⅲ

题目链接：https://leetcode.cn/problems/best-time-to-buy-and-sell-stock-iii/description/

class Solution {
public:
    const int INF = 0x3f3f3f3f;
    int maxProfit(vector<int>& prices) 
    {
        /*时空复杂度O(n)*/
        int n = prices.size();
        // 1. 创建dp表
        vector<vector<int>> f(n, vector<int>(3, -INF));
        auto g = f;
        // 2. 初始化
        f[0][0] = -prices[0], g[0][0] = 0;
        // 3. 填表
        for (int i = 1; i < n; ++i)
            for (int j = 0; j < 3; ++j)
            {
                f[i][j] = max(f[i - 1][j], g[i - 1][j] - prices[i]);
                g[i][j] = g[i - 1][j];
                if (j >= 1) g[i][j] = max(g[i - 1][j], f[i - 1][j - 1] + prices[i]);
            }
        // 4. 返回值
        // return max(g[n - 1][0], max(g[n - 1][1], g[n - 1][2]));
        int ret = 0;
        for (int j = 0; j < 3; ++j)
            ret = max(ret, g[n - 1][j]);
        return ret;
    }
};