Valid Palindrome (#125 Leetcode)

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.

 

class Solution {
public:
    bool isPalindrome(string s) {
        if (s.empty()) { return true; }
        int j = s.size();
        int i = 0;
        while (i < j) {
            /* Numbers 0 to 9 are allowed, so ASCII values 
               49 to 57 are allowed. Alphabets 'a' to 'z' are allowed,
               so 97 to 122 are allowed*/
            if (!isalnum(s[i])) { i++; continue; } 
            if (!isalnum(s[j])) { j--; continue; } 
            
            if (tolower(s[i++]) != tolower(s[j--])) {
                return false;
            }
        }  
        return true;
    }
};