125. Valid Palindrome

• Difficulty: Easy

• Topics: Two Pointers, String

• Similar Questions:

  • Palindrome Linked List

  • Valid Palindrome II

Problem:

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

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input: "A man, a plan, a canal: Panama"
Output: true

Example 2:

Input: "race a car"
Output: false

Solutions:

class Solution {
public:
    bool isPalindrome(string s) {
        int left = 0;
        int right = s.length() - 1;

        while (left < right) {
            while (left < right && !isalphanum(s[left])) {
                ++left;
            }

            while (left < right && !std::isalnum(s[right])){ // remember the name of the funciton
                --right;
            }

            if (s[left] >= '0' && s[left] <= '9' && s[left] != s[right]) {
                return false;
            }

            if (tolower(s[left]) != tolower(s[right])) {
                return false;
            }


            ++left;
            --right;
        }

        return true;
    }

    bool isalphanum(char c) {
        return c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c >= '0' && c <= '9';
    }

};