LeetCode 32 Longest Valid Parentheses (C++, Python)

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Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.
For "(()", the longest valid parentheses substring is "()", which has length = 2.
Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.

Solution C++


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class Solution {
public:
    int longestValidParentheses(string s) {
        int left = 0;
        int right = 0;
        int maxlen = 0;
        for (int i = 0; i < s.length(); ++i) {
            if (s[i] == '(') {
                left ++;
            } else {
                right ++;
            }
            if (left < right) {
                left = 0;
                right = 0;
            } else if (left == right) {
                maxlen = max(maxlen, right * 2);
            }
        }
        left = 0;
        right = 0;
        for (int i = s.length() - 1; i >= 0; --i) {
            if (s[i] == '(') {
                left ++;
            } else {
                right ++;
            }
            if (left > right) {
                left = 0;
                right = 0;
            } else if (left == right) {
                maxlen = max(maxlen, right * 2);
            }
        }
        return maxlen;
    }
};
Solution Python


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class Solution:
    # @param s, a string
    # @return an integer
    def longestValidParentheses(self, s):
        dp = [0] * (len(s) + 1)
        maxlen = 0
        for i in range(len(s) - 1, -1, -1):
            if s[i] == ')': continue
            j = i + 1 + dp[i + 1]
            if j == len(s): continue
            if s[j] == '(': continue
            dp[i] = 2 + dp[i + 1] + dp[j + 1]
            maxlen = max(maxlen, dp[i])
        return maxlen

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