Description

Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward.

Example 1:

Input: 121
Output: true

Example 2:

Input: -121
Output: false
Explanation: From left to right, it reads -121. From right to left, it becomes 121-. Therefore it is not a palindrome.

Example 3:

Input: 10
Output: false
Explanation: Reads 01 from right to left. Therefore it is not a palindrome.

Follow up:

Could you solve it without converting the integer to a string?

Submission

Simple but Not Meet the Requirement

A straightforward solution is to convert the number into a string and then from both sides check if the string is symmetric.

class Solution {
    public boolean isPalindrome(int x) {
        String s = Integer.toString(x);
        if(s.length() <= 1) return true;
        for(int i = 0; i <= s.length() / 2; i++) {
            if(s.charAt(i) != s.charAt(s.length() - 1 - i)) return false;
        }
        
        return true;
    }
}

The Desired Answer

The following one is written in an Integer manner, slightly faster than the previous one and does not convert the number to a string prior to the comparison.

class Solution {
    public boolean isPalindrome(int x) {
        if(x < 0 || (x % 10 == 0 && x != 0)) return false;
        
        int reversed = 0;
        while(x > reversed) {
            reversed = reversed * 10 + x % 10;
            x /= 10;
        }
        
        return x == reversed || x == reversed / 10;
    }
}