Change 'len' to 'length'

Author: YingfanGu

problems/0121.买卖股票的最佳时机.md

@@ -308,7 +308,7 @@ class Solution:
 class Solution:
     def maxProfit(self, prices: List[int]) -> int:
         length = len(prices)
-        if len == 0:
+        if length == 0:
             return 0
         dp = [[0] * 2 for _ in range(length)]
         dp[0][0] = -prices[0]