309. Best Time to Buy and Sell Stock with Cooldown.cpp

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

// Since day `i` relies only on `i-1`, we can reduce the O(n) space to O(1). 
class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int n = prices.size();
        if(n < 2) return 0;
        int buy = -prices[0], sell = 0, rest = 0, preBuy, preSell;
        for(int i = 1; i < n; i++){
            preBuy = buy;
            preSell = sell;
            buy = max(buy, rest - prices[i]);
            sell = max(sell, preBuy + prices[i]);
            rest = max(rest, preSell);
        }
        return max(rest, sell);
    }
};