update

Author: jasoncao11

P101. 最长有效括号.md

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+***给你一个只包含 '(' 和 ')' 的字符串，找出最长有效（格式正确且连续）括号子串的长度。***
+
+![algo39](./images/algo39.jpg)
+![algo40](./images/algo40.jpg)
+
+```
+class Solution:
+    def longestValidParentheses(self, s: str) -> int:
+        if not s:
+            return 0
+        n = len(s)
+        dp = [0]*n
+
+        for i in range(1,n):
+            if s[i] == ')':
+                if s[i-1] == '(':
+                    dp[i] = dp[i-2]+2 if (i-2) >= 0 else 2
+                else:
+                    if (i-dp[i-1]-1) >= 0 and s[i-dp[i-1]-1] == '(':
+                        dp[i] = dp[i-1]+2
+                        if (i-dp[i-1]-2) >= 0:
+                            dp[i] += dp[i-dp[i-1]-2]
+        return max(dp)
+```