121. Best Time to Buy and Sell Stock

You are given an array prices where prices[i] is the price of a given stock on the ith day.

You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock.

Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.

代码：

class Solution {
    public int maxProfit(int[] prices) {
        int min = prices[0];
        int maxProfit = 0;
        for (int i = 1; i < prices.length; i++) {
            if (prices[i] <= min) {
                min = prices[i];
            } else {
                maxProfit = Math.max(maxProfit, prices[i] - min);
            }
        }
        return maxProfit;
    }
}

122. Best Time to Buy and Sell Stock II

You are given an integer array prices where prices[i] is the price of a given stock on the ith day.

On each day, you may decide to buy and/or sell the stock. You can only hold at most one share of the stock at any time. However, you can buy it then immediately sell it on the same day.

Find and return the maximum profit you can achieve.

题目解析：

• 将递增的时间段的首尾作为买入和卖出的点

代码：

class Solution {
    public int maxProfit(int[] prices) {
        int min = prices[0];
        int max = prices[0];
        int profit = 0;
        for (int i = 1; i < prices.length; i++) {
            if (prices[i] >= max) {
                max = prices[i];
            } else {
                profit += (max - min);
                min = prices[i];
                max = prices[i];
            }
        }

        return profit + max - min;
    }
}