121. 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.

这题有O（N）解，因为情况限定在了后面减去前面的最大差。

class Solution {
    public int maxProfit(int[] prices) {
        int small = Integer.MAX_VALUE;
        int result = 0 ;
        for(int i = 0 ;iresult)
                      result=prices[i]-small;
            }
        }
        return result ;
    }
}