Best Time to Buy and Sell Stock

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.

/*
Every day, we should record the current lowest price and only need to compute the profit at this day based on the current lowest price. If the profit is bigger than the previous max profit, update the max profit. 

time:O(n), space:O(1)
*/

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