Question

125. Valid Palindrome

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Description

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string s, return true_ if it is a palindrome, or _false_ otherwise_.

 

Example 1:

Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.

Example 2:

Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.

Example 3:

Input: s = " "
Output: true
Explanation: s is an empty string "" after removing non-alphanumeric characters.
Since an empty string reads the same forward and backward, it is a palindrome.

 

Constraints:

• 1 <= s.length <= 2 * 105
• s consists only of printable ASCII characters.

Solutions

Solution 1: Two Pointers

We use two pointers $i$ and $j$ to point to the two ends of the string $s$, and then loop through the following process until $i \geq j$:

1. If $s[i]$ is not a letter or a number, move the pointer $i$ one step to the right and continue to the next loop.
2. If $s[j]$ is not a letter or a number, move the pointer $j$ one step to the left and continue to the next loop.
3. If the lowercase form of $s[i]$ and $s[j]$ are not equal, return false.
4. Otherwise, move the pointer $i$ one step to the right and the pointer $j$ one step to the left, and continue to the next loop.

At the end of the loop, return true.

The time complexity is $O(n)$, where $n$ is the length of the string $s$. The space complexity is $O(1)$.

class Solution:
  def isPalindrome(self, s: str) -> bool:
    i, j = 0, len(s) - 1
    while i < j:
      if not s[i].isalnum():
        i += 1
      elif not s[j].isalnum():
        j -= 1
      elif s[i].lower() != s[j].lower():
        return False
      else:
        i, j = i + 1, j - 1
    return True

class Solution {
  public boolean isPalindrome(String s) {
    int i = 0, j = s.length() - 1;
    while (i < j) {
      if (!Character.isLetterOrDigit(s.charAt(i))) {
        ++i;
      } else if (!Character.isLetterOrDigit(s.charAt(j))) {
        --j;
      } else if (Character.toLowerCase(s.charAt(i)) != Character.toLowerCase(s.charAt(j))) {
        return false;
      } else {
        ++i;
        --j;
      }
    }
    return true;
  }
}

class Solution {
public:
  bool isPalindrome(string s) {
    int i = 0, j = s.size() - 1;
    while (i < j) {
      if (!isalnum(s[i])) {
        ++i;
      } else if (!isalnum(s[j])) {
        --j;
      } else if (tolower(s[i]) != tolower(s[j])) {
        return false;
      } else {
        ++i;
        --j;
      }
    }
    return true;
  }
};

func isPalindrome(s string) bool {
  i, j := 0, len(s)-1
  for i < j {
    if !isalnum(s[i]) {
      i++
    } else if !isalnum(s[j]) {
      j--
    } else if tolower(s[i]) != tolower(s[j]) {
      return false
    } else {
      i, j = i+1, j-1
    }
  }
  return true
}

func isalnum(ch byte) bool {
  return (ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z') || (ch >= '0' && ch <= '9')
}

func tolower(ch byte) byte {
  if ch >= 'A' && ch <= 'Z' {
    return ch + 32
  }
  return ch
}

function isPalindrome(s: string): boolean {
  let i = 0;
  let j = s.length - 1;
  while (i < j) {
    if (!/[a-zA-Z0-9]/.test(s[i])) {
      ++i;
    } else if (!/[a-zA-Z0-9]/.test(s[j])) {
      --j;
    } else if (s[i].toLowerCase() !== s[j].toLowerCase()) {
      return false;
    } else {
      ++i;
      --j;
    }
  }
  return true;
}

impl Solution {
  pub fn is_palindrome(s: String) -> bool {
    let s = s.to_lowercase();
    let s = s.as_bytes();
    let n = s.len();
    let (mut l, mut r) = (0, n - 1);
    while l < r {
      while l < r && !s[l].is_ascii_alphanumeric() {
        l += 1;
      }
      while l < r && !s[r].is_ascii_alphanumeric() {
        r -= 1;
      }
      if s[l] != s[r] {
        return false;
      }
      l += 1;
      if r != 0 {
        r -= 1;
      }
    }
    true
  }
}

/**
 * @param {string} s
 * @return {boolean}
 */
var isPalindrome = function (s) {
  let i = 0;
  let j = s.length - 1;
  while (i < j) {
    if (!/[a-zA-Z0-9]/.test(s[i])) {
      ++i;
    } else if (!/[a-zA-Z0-9]/.test(s[j])) {
      --j;
    } else if (s[i].toLowerCase() !== s[j].toLowerCase()) {
      return false;
    } else {
      ++i;
      --j;
    }
  }
  return true;
};

public class Solution {
  public bool IsPalindrome(string s) {
    int i = 0, j = s.Length - 1;
    while (i < j) {
      if (!char.IsLetterOrDigit(s[i])) {
        ++i;
      } else if (!char.IsLetterOrDigit(s[j])) {
        --j;
      } else if (char.ToLower(s[i++]) != char.ToLower(s[j--])) {
        return false;
      }
    }
    return true;
  }
}

class Solution {
  /**
   * @param String $s
   * @return Boolean
   */
  function isPalindrome($s) {
    $regex = '/[a-z0-9]/';
    $s = strtolower($s);
    preg_match_all($regex, $s, $matches);
    if ($matches[0] == null) {
      return true;
    }
    $len = floor(count($matches[0]) / 2);
    for ($i = 0; $i < $len; $i++) {
      if ($matches[0][$i] != $matches[0][count($matches[0]) - 1 - $i]) {
        return false;
      }
    }
    return true;
  }
}