Problem: Palindrome Number

Introduction to DSA & Problem Solving – Problem: Palindrome Number

Problem Statement

Write a function isPalindrome(x) that takes an integer x and returns true if it reads the same backward and forward; otherwise, returns false.

Requirements

• Must handle both positive and negative integers.
• Return false for negative numbers (since -121 ≠ 121-).

Constraints

Examples

Step-by-Step Approach

1. Handle Negatives
   If x < 0, return false.
2. Store Original Value
   Keep a copy of the original number → xCopy = x.
3. Reverse the Number
   • Initialize rev = 0
   • While x > 0:
     • rem = x % 10
     • rev = rev * 10 + rem
     • x = Math.floor(x / 10)
4. Compare Values
   • If rev === xCopy, return true
   • Otherwise, return false

Visualization

x = 121
rev = 0

Step 1 → rem = 1, rev = 0 * 10 + 1 = 1
Step 2 → rem = 2, rev = 1 * 10 + 2 = 12
Step 3 → rem = 1, rev = 12 * 10 + 1 = 121

rev === xCopy → 121 === 121 → true

Flowchart

JavaScript Code

var isPalindrome = function(x) {
  
  // Handle Negatives
  if (x < 0) return false;
  
  // Store Original Value
  let xCopy = x;
  let rev = 0;
  
  // Reverse the Number
  while (x > 0) {
    let rem = x % 10;
    rev = rev * 10 + rem;
    x = Math.floor(x / 10);
  }
  
  // Compare Values
  return rev === xCopy;
};

console.log(isPalindrome(121)); // true

Key Takeaways

• Negative integers can never be palindromes.
• Efficient O(1) space and O(d) time.
• Works purely mathematically—no string conversion needed.