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 (i.e., buy one and sell one share of the stock), design an algorithm to find the maximum profit.

Note that you cannot sell a stock before you buy one.

Example 1:

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.
Example 2:

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

遍历一遍，期间维护两个值，一个是当前最大值，一个是遍历过的数字中的最小值，则最新的最大值会产生在当前最大值与（当前数字减去最小值）之间，不断更新。

public class Solution {
    public int MaxProfit(int[] prices) {
        int result = 0;
        int min = (prices.Length>0) ? prices[0] : 0 ;   //记录遍历过的最小值
        
        for (int i = 0; i < prices.Length; ++i) {
            min = Math.Min(min, prices[i]);
            result = Math.Max(result, prices[i]-min);   //对比当前最大值 vs 新增可能最大值
        }
        return result;
    }
}