# Valid Palindrome: converting uppercase to lowercase and removing non-alphanumeric characters

2.42K views

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
“A man, a plan, a canal: Panama” is a palindrome. “race a car” is not a palindrome.

Q: What about an empty string? Is it a valid palindrome?
A: For the purpose of this problem, we define empty string as valid palindrome.

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.```

Farjanul Nayem Answered question July 27, 2022
0

O(n) runtime, O(1) space:

The idea is simple, have two pointers – one at the head while the other one at the tail. Move them towards each other until they meet while skipping non-alphanumeric characters.

Consider the case where given string contains only non-alphanumeric characters. This is a valid palindrome because the empty string is also a valid palindrome.

Solution in Java

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

Solution in Python

```class Solution(object):
def isPalindrome(self, s):
"""
:type s: str
:rtype: bool
"""
alnum_s = [t.lower() for t in s if t.isalnum()]
ls = len(alnum_s)
if ls <= 1:
return True
mid = ls / 2
for i in range(mid):
if alnum_s[i] != alnum_s[ls - 1 - i]:
return False
return True```

Farjanul Nayem Answered question July 27, 2022
0