125. Valid Palindrome

125. Valid Palindrome (Easy)

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

Companies:
Facebook, Microsoft, Apple

Related Topics:
Two Pointers, String

Similar Questions:

• Palindrome Linked List (Easy)
• Valid Palindrome II (Easy)

Solution 1.

// OJ: https://leetcode.com/problems/valid-palindrome/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    bool isPalindrome(string s) {
        int i = 0, j = s.size();
        while (i < j) {
            while (i < j && !isalnum(s[i])) ++i;
            while (i < j && !isalnum(s[j])) --j;
            if (i < j && tolower(s[i++]) != tolower(s[j--])) return false;
        }
        return true;
    }
};