LeetCode #9: Palindrome Number

The Problem

Given an integer x, return true if it is a palindrome, and false otherwise.

A palindrome reads the same forwards and backwards.

Examples

Key Insight

• Negative numbers are never palindromes because of the minus sign
• Single-digit numbers (0–9) are always palindromes
• You can reverse the number mathematically and compare
• Don’t convert to string — that uses extra space!

✅ Simple & Efficient Solution

Reverse the number digit by digit and compare with the original.

function isPalindrome(x) {
  // Negative numbers and numbers ending in 0 (but not 0) can't be palindromes
  if (x < 0 || (x % 10 === 0 && x !== 0)) return false;

  let reversed = 0;
  let temp = x;

  while (temp > 0) {
    reversed = reversed * 10 + (temp % 10);
    temp = Math.trunc(temp / 10);
  }

  return reversed === x;
}

How It Works

1. Reject negatives and numbers like 10, 100 (they end in 0 but aren‘t single-digit)
2. Reverse the number using:
   • digit = temp % 10 → get last digit
   • reversed = reversed * 10 + digit → build reversed number
   • temp = Math.trunc(temp / 10) → remove last digit
3. Compare reversed with original

💡 Bonus: String Approach (Not Recommended)

You can solve this in one line — but it‘s not optimal.

function isPalindrome(x) {
  return x >= 0 && x.toString() === x.toString().split('').reverse().join('');
}

⚠️ Why avoid it? It uses O(d) extra space (where d = digits), and string conversion is slower.