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20. Valid Parentheses 👍

  • Time: $O(n)$
  • Space: $O(n)$
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class Solution {
 public:
  bool isValid(string s) {
    stack<char> stack;

    for (const char c : s)
      if (c == '(')
        stack.push(')');
      else if (c == '{')
        stack.push('}');
      else if (c == '[')
        stack.push(']');
      else if (stack.empty() || pop(stack) != c)
        return false;

    return stack.empty();
  }

 private:
  int pop(stack<char>& stack) {
    const int c = stack.top();
    stack.pop();
    return c;
  }
};

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class Solution {
  public boolean isValid(String s) {
    Deque<Character> stack = new ArrayDeque<>();

    for (final char c : s.toCharArray())
      if (c == '(')
        stack.push(')');
      else if (c == '{')
        stack.push('}');
      else if (c == '[')
        stack.push(']');
      else if (stack.isEmpty() || stack.pop() != c)
        return false;

    return stack.isEmpty();
  }
}

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class Solution:
  def isValid(self, s: str) -> bool:
    stack = []

    for c in s:
      if c == '(':
        stack.append(')')
      elif c == '{':
        stack.append('}')
      elif c == '[':
        stack.append(']')
      elif not stack or stack.pop() != c:
        return False

    return not stack