Longest Valid Parentheses LeetCode solution in C++

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class Solution {
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 public:
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  int longestValidParentheses(string s) {
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    int n = s.length(), i = 0, ans = 0, k = 0;
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    for (int j = 0; j < n; j++) {
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      if (s[j] == '(')
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        k++;
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      else if (s[j] == ')') {
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        k--;
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        if (k == 0) ans = max(ans, j - i + 1);
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      }
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      if (k < 0) {
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        k = 0;
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        i = j + 1;
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      }
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    }
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    k = 0, i = n - 1;
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    for (int j = n - 1; j >= 0; j--) {
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      if (s[j] == ')') {
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        k++;
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      } else if (s[j] == '(') {
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        k--;
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        if (k == 0) ans = max(ans, i - j + 1);
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      }
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      if (k < 0) {
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        k = 0;
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        i = j - 1;
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      }
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    }
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    return ans;
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  }
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};