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125. Valid Palindrome

  • Time: $O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  bool isPalindrome(string s) {
    int l = 0;
    int r = s.length() - 1;

    while (l < r) {
      while (l < r && !isalnum(s[l]))
        ++l;
      while (l < r && !isalnum(s[r]))
        --r;
      if (tolower(s[l]) != tolower(s[r]))
        return false;
      ++l;
      --r;
    }

    return true;
  }
};

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class Solution {
  public boolean isPalindrome(String s) {
    int l = 0;
    int r = s.length() - 1;

    while (l < r) {
      while (l < r && !Character.isLetterOrDigit(s.charAt(l)))
        ++l;
      while (l < r && !Character.isLetterOrDigit(s.charAt(r)))
        --r;
      if (Character.toLowerCase(s.charAt(l)) != Character.toLowerCase(s.charAt(r)))
        return false;
      ++l;
      --r;
    }

    return true;
  }
}

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class Solution:
  def isPalindrome(self, s: str) -> bool:
    l = 0
    r = len(s) - 1

    while l < r:
      while l < r and not s[l].isalnum():
        l += 1
      while l < r and not s[r].isalnum():
        r -= 1
      if s[l].lower() != s[r].lower():
        return False
      l += 1
      r -= 1

    return True

var isPalindrome = function(s) {
  let l = 0, r = s.length - 1;
  const ok = (c) => /[a-z0-9]/i.test(c);
  while (l < r) {
    while (l < r && !ok(s[l])) l++;
    while (l < r && !ok(s[r])) r--;
    if (s[l].toLowerCase() !== s[r].toLowerCase()) return false;
    l++;
    r--;
  }
  return true;
};
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