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

class Solution {
public:
    int longestValidParentheses(string s) {
        int l = s.length();
        if (l < 2)return 0;
        int ans = 0;
        vector<int>dp;
        dp.resize(l);
        for (int i = 0; i < l; i++)
            dp[i] = 0;
        if (s[0] == '('&&s[1] == ')') {
            ans = 2;
            dp[1] = 2;
        }
        for (int i = 2; i < l; i++) {
            if (s[i] == '(')continue;
            if (s[i - 1] == '(')dp[i] = dp[i - 2] + 2;
            else {
                int last = dp[i - 1];
                if (i - last - 1 >= 0 && s[i - last - 1] == '(') {
                    dp[i] = last + 2;
                    if (i - last - 2 >= 0)
                        dp[i] += dp[i - last - 2];
                }
            }
            ans = max(ans, dp[i]);
        }
        return ans;
    }
};