[LeetCode] Best Time to Buy and Sell Stock III

Personally, this is a relatively difficult DP problem. This link posts a typical DP solution to it. You may need some time to get how it works.

The code is rewritten as follows.

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

Personally, I prefer this solution, which is much easier to understand. The code is also rewritten as follows.

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int states[2][4] = {INT_MIN, 0, INT_MIN, 0};
        int n = prices.size(), cur = 0, next = 1;
        for (int i = 0; i < n; i++) {
            states[next][0] = max(states[cur][0], -prices[i]);
            states[next][1] = max(states[cur][1], states[cur][0] + prices[i]);
            states[next][2] = max(states[cur][2], states[cur][1] - prices[i]);
            states[next][3] = max(states[cur][3], states[cur][2] + prices[i]);
            swap(cur, next);
        }
        return max(states[cur][1], states[cur][3]);
    }
};