LeetCode-in-Java

9. Palindrome Number

Easy

Given an integer x, return true if x is palindrome integer.

An integer is a palindrome when it reads the same backward as forward. For example, 121 is palindrome while 123 is not.

Example 1:

Input: x = 121

Output: true

Example 2:

Input: x = -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: x = 10

Output: false

Explanation: Reads 01 from right to left. Therefore it is not a palindrome.

Example 4:

Input: x = -101

Output: false

Constraints:

• -231 <= x <= 231 - 1

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

To solve the Palindrome Number problem in Java using a Solution class, we’ll follow these steps:

1. Define a Solution class with a method named isPalindrome.
2. Handle special cases where x is negative or divisible by 10 but not equal to zero, as they cannot be palindromes.
3. Initialize variables to keep track of the reversed number (reversed) and the original number (original).
4. Iterate through the digits of the original number x:
   • Extract the least significant digit using the modulo operator and append it to the reversed number.
   • Update original by removing the least significant digit using integer division.
5. Check if the reversed number is equal to the original number. If so, return true; otherwise, return false.

Here’s the implementation:

public class Solution {

    public boolean isPalindrome(int x) {
        // Special cases:
        // As discussed, negative numbers are not palindromes.
        // Also, if the last digit of the number is 0, it can't be a palindrome unless the number is 0 itself.
        if (x < 0 || (x % 10 == 0 && x != 0)) {
            return false;
        }

        int reversed = 0;
        int original = x;

        while (original > reversed) {
            int digit = original % 10;
            reversed = reversed * 10 + digit;
            original /= 10;
        }

        // When the length is an odd number, we can get rid of the middle digit by reversed / 10
        // For example when the input is 12321, at the end of the while loop we get x = 12, reversed = 123,
        // since the middle digit doesn't matter in palidrome(it will always equal to itself), we can simply get rid of it.
        return original == reversed || original == reversed / 10;
    }

    public static void main(String[] args) {
        Solution solution = new Solution();

        // Test cases
        int x1 = 121;
        System.out.println("Example 1 Output: " + solution.isPalindrome(x1));

        int x2 = -121;
        System.out.println("Example 2 Output: " + solution.isPalindrome(x2));

        int x3 = 10;
        System.out.println("Example 3 Output: " + solution.isPalindrome(x3));

        int x4 = -101;
        System.out.println("Example 4 Output: " + solution.isPalindrome(x4));
    }
}

This implementation provides a solution to the Palindrome Number problem in Java.

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