125. Valid Palindrome

1. 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.

2. Example

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.

3. Constraints

• 1 <= s.length <= 2 * $10^5$
• s consists only of printable ASCII characters.

4. Solutions

Two Pointers

n = str.size()
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    bool isPalindrome(const string &str) {
        const int n = str.size();
        for (int left = 0, right = n - 1; left < right; ++left, --right) {
            while (left < right && !isalnum(str[left])) {
                ++left;
            }
            while (left < right && !isalnum(str[right])) {
                --right;
            }

            if (left < right && tolower(str[left]) != tolower(str[right])) {
                return false;
            }
        }

        return true;
    }
};