Update 121._Best_Time_to_Buy_and_Sell_Stock.md

121._Best_Time_to_Buy_and_Sell_Stock.md

@@ -19,7 +19,8 @@ Here, the logic is to calculate the difference ```(maxCur += prices[i] - prices[
 of the original array, and find a contiguous subarray giving maximum profit. 
 If the difference falls below ```0```, reset it to zero.
 
-
+参考[Maximum subarray problem](https://en.wikipedia.org/wiki/Maximum_subarray_problem), 
+[Kadane's Algorithm](https://discuss.leetcode.com/topic/19853/kadane-s-algorithm-since-no-one-has-mentioned-about-this-so-far-in-case-if-interviewer-twists-the-input)
 ```python
 class Solution(object):
     def maxProfit(self, prices):