121. Best Time to Buy and Sell Stock

Say you have an array for which theithelement is the price of a given stock on dayi.

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.

遍历价格数组，找到截止到当前点为止的最小价格，并且算出截止到当前点最大利润。如果用趋势图的思维，就是找历史最低点和历史最高点。

class Solution {
    public int maxProfit(int[] prices) {
        if (prices.length < 2) {
            return 0;
        }
        int maxProfit = 0;
        int minPrice = prices[0];
        for (int i = 0; i < prices.length; i++) {
            maxProfit = Math.max(maxProfit, prices[i] - minPrice);
            minPrice = Math.min(minPrice, prices[i]);
        }
        return maxProfit;
    }
}