309. Best Time to Buy and Sell Stock with Cooldown

DP有点难。
看到一个不错的讲解，对应的转移方程：

• 在持有 buy[i] = max(sell[i-2] - price[i], buy[i-1])

• 不在持有 sell[i] = max(buy[i-1] + price[i],sell[i-1])

代码的话像覃超说的，转移方程出来了就不要动脑子了。注意Java这边不太好用-1做数组下标。

    public int maxProfit(int[] prices) {
        if (prices.length < 2) return 0;
        int[] buy = new int[prices.length];
        int[] sell = new int[prices.length];
        buy[0] = -prices[0];
        buy[1] = Math.max(-prices[0] , -prices[1]);
        sell[0] = 0;
        sell[1] = Math.max(prices[1] - prices[0] , 0 );
        for (int i = 2; i < prices.length; i++) {
            sell[i] = Math.max(buy[i - 1] + prices[i], sell[i - 1]);
            buy[i] = Math.max(sell[i - 2] - prices[i], buy[i - 1]);
        }
        return sell[sell.length - 1];
    }
}