# 125. Valid Palindrome

## 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 {
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;
}
};

• 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


• 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;
}


• /**
* @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;
};


• 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;
}
}

• 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;
}
}

• 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
}
}