121. Best Time to Buy and Sell Stock

Input: [7,1,5,3,6,4]
Output: 5
Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5.
             Not 7-1 = 6, as selling price needs to be larger than buying price.

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

class Solution {
    public int maxProfit(int[] prices) {
        if (prices == null || prices.length < 2) return 0;
        int min = prices[0];
        int profit = 0;
        for (int price : prices) {
            min = Math.min(min, price);
            profit = Math.max(profit, price - min);
        }
        return profit;
    }
}