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32. Longest Valid Parentheses 👍

  • Time: $O(n)$
  • Space: $O(n)$
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class Solution {
 public:
  int longestValidParentheses(string s) {
    const string s2 = ")" + s;
    // dp[i] := Length of longest valid parentheses substring of s2[1..i]
    vector<int> dp(s2.length());

    for (int i = 1; i < s2.length(); ++i)
      if (s2[i] == ')' && s2[i - dp[i - 1] - 1] == '(')
        dp[i] = dp[i - 1] + dp[i - dp[i - 1] - 2] + 2;

    return *max_element(dp.begin(), dp.end());
  }
};

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class Solution {
  public int longestValidParentheses(String s) {
    final String s2 = ")" + s;
    // dp[i] := Length of longest valid parentheses substring of s2[1..i]
    int dp[] = new int[s2.length()];

    for (int i = 1; i < s2.length(); ++i)
      if (s2.charAt(i) == ')' && s2.charAt(i - dp[i - 1] - 1) == '(')
        dp[i] = dp[i - 1] + dp[i - dp[i - 1] - 2] + 2;

    return Arrays.stream(dp).max().getAsInt();
  }
}

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class Solution:
  def longestValidParentheses(self, s: str) -> int:
    s2 = ')' + s
    # dp[i] := Length of longest valid parentheses substring of s2[1..i]
    dp = [0] * len(s2)

    for i in range(1, len(s2)):
      if s2[i] == ')' and s2[i - dp[i - 1] - 1] == '(':
        dp[i] = dp[i - 1] + dp[i - dp[i - 1] - 2] + 2

    return max(dp)