# Valid Palindrome Problem

## Description

LeetCode Problem 125.

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:

``````1
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Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.
``````

Example 2:

``````1
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Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.
``````

Example 3:

``````1
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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 * 10^5
• s consists only of printable ASCII characters.

## Sample C++ Code

``````1
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bool isPalindrome(string s) {
for (int i = 0, j = s.size() - 1; i < j; i++, j--) {
// Move 2 pointers from each end until they collide
while (!isalnum(s[i]) && i < j)
// Increment left pointer if not alphanumeric
i++;
while (!isalnum(s[j]) == false && i < j)
// Decrement right pointer if no alphanumeric
j--;
if (toupper(s[i]) != toupper(s[j]))
// Exit and return error if not match
return false;
}

return true;
}
``````