LeetCode-121：Best Time to Buy and Sell Stock (一次股票交易最大利润)

Question

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

If you were only permitted to complete at most one transaction (ie, buy one and sell one share of the stock), design an algorithm to find the maximum profit.

Example 1：

Input: [7, 1, 5, 3, 6, 4]
Output: 5
max. difference = 6-1 = 5 (not 7-1 = 6, as selling price needs to be larger than buying price)

Example 2：

Input: [7, 6, 4, 3, 1]
Output: 0
In this case, no transaction is done, i.e. max profit = 0.

Idea

给定一个数组，其中的第i个元素为第i天给出的股票价格，求出一次交易利润。

对于这类问题，只需要对数组进行一次遍历，在遍历过程中记录利润，并在循环过程中保留最大利润即可。

Code

一次遍历—Java

public class Solution {
    public int maxProfit(int[] prices) {
        if(prices == null || prices.length == 0){
            return 0;
        }
        int buy = prices[0];
        int max = 0;
        for (int i=0; i<prices.length; i++){
            if (prices[i] < buy){
                buy = prices[i];
            }else{
                int profit = prices[i]-buy;
                if (max < profit){
                    max = profit;
                }
            }
        }
        return max;
    }
}

• 处理过程中，要考虑数组为null的情况；
• 这里在遍历的过程中，一旦遇到价格大于买入价格，就计算利润，保留最大利润，避免两次for循环；
• 时间复杂度：O(n)，空间复杂度：O(1)