买卖股票的最佳时机

LC121：最多只允许完成一笔交易

dp[i]表示前i天所能获得的最大利润

    public int maxProfit(int[] prices) {
        int len = prices.length;
        if (len <= 1) {
            return 0;
        }
        int[] dp = new int[len];
        dp[0] = 0;
        int minPrice = prices[0];
        for (int i = 1; i < len; i++) {
            if (minPrice > prices[i]) {
                minPrice = prices[i];
            }
            dp[i] = Math.max(prices[i] - minPrice, dp[i - 1]);
        }
        return dp[len - 1];
    }

LC122：可以多次交易买卖

    public int maxProfit(int[] prices) {
        int profit = 0;
        for (int i = 1; i < prices.length; i++) {
            int tmp = prices[i] - prices[i - 1];
            if (tmp > 0) profit += tmp;
        }
        return profit;
    }