121. Best Time to Buy and Sell Stock

Problem:

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.

Analysis:

自己写出来的one pass AC。还是不错，对比4年前写的，进步非常大。

Solution:

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