19.2.1 [LeetCode 32] Longest Valid Parentheses

Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

Example 1:

Input: "(()"
Output: 2
Explanation: The longest valid parentheses substring is "()"

Example 2:

Input: ")()())"
Output: 4
Explanation: The longest valid parentheses substring is "()()"

题意

找最长的合法左右括号字符串

题解

这道题还蛮难的，一开始我的dp思路错误了（n3的复杂度真是不像话）

把dp数组设为以当前索引值为末尾的合法字符串的长度就可以做了

 1 class Solution {
 2 public:
 3     int longestValidParentheses(string s) {
 4         int l = s.length();
 5         if (l < 2)return 0;
 6         int ans = 0;
 7         vector<int>dp;
 8         dp.resize(l);
 9         for (int i = 0; i < l; i++)
10             dp[i] = 0;
11         if (s[0] == '('&&s[1] == ')') {
12             ans = 2;
13             dp[1] = 2;
14         }
15         for (int i = 2; i < l; i++) {
16             if (s[i] == '(')continue;
17             if (s[i - 1] == '(')dp[i] = dp[i - 2] + 2;
18             else {
19                 int last = dp[i - 1];
20                 if (i - last - 1 >= 0 && s[i - last - 1] == '(') {
21                     dp[i] = last + 2;
22                     if (i - last - 2 >= 0)
23                         dp[i] += dp[i - last - 2];
24                 }
25             }
26             ans = max(ans, dp[i]);
27         }
28         return ans;
29     }
30 };

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