122. Best Time to Buy and Sell Stock II

Description

Say you have an array for which the ith element is the price of a given stock on day i.

Design an algorithm to find the maximum profit. You may complete as many transactions as you like (ie, buy one and sell one share of the stock multiple times). However, you may not engage in multiple transactions at the same time (ie, you must sell the stock before you buy again).

Solution

Greedy, time O(n), space O(1)

定义delta[i] = prices[i + 1] - prices[i]
假设prices[y] - prices[x]代表在x天买入y天卖出，假设delta[y + 1] > 0，则prices[y + 1] - prices[x]一定更优。根据这个思路可以用Greedy求解。

class Solution {
    public int maxProfit(int[] prices) {
        if (prices == null || prices.length < 2) {
            return 0;
        }    
        
        int maxProfit = 0;

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