Palindrome Number

Problem

Given an integer x, return true if it is a palindrome (reads the same forwards and backwards). Negative numbers are not palindromes.

Constraints

• −2³¹ ≤ x ≤ 2³¹ − 1

Approach — Core Patterns

This is a Core Patterns problem. The idea: pick the data structure best matched to the constraints and solve it in one clean pass. Work through the reference code below line by line, then re-derive it yourself in the editor — that's how the pattern sticks.

Complexity: see the walkthrough below.

Solution code

Python

class Solution:
    def isPalindrome(self, x):
        if x < 0:
            return False
        return str(x) == str(x)[::-1]

Java

class Solution {
    public boolean isPalindrome(int x) {
        // Negatives and any number ending in 0 (besides 0 itself) can't be palindromes.
        if (x < 0 || (x % 10 == 0 && x != 0)) {
            return false;
        }
        int reversed = 0;
        while (x > reversed) {
            reversed = reversed * 10 + x % 10;
            x /= 10;
        }
        // Even length → x == reversed; odd length → middle digit is in reversed,
        // so drop it with reversed / 10.
        return x == reversed || x == reversed / 10;
    }
}

Practice it

Reading a solution isn't the same as being able to write it under pressure. Open this problem in the in-browser editor, hide the solution, and solve it from scratch — your code runs against real test cases instantly.