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🔁 Palindrome Number in JavaScript

A palindrome is a word, phrase, or number that reads the same forward and backward.

✨ Examples:

• Words: madam, level, radar

• Numbers: 121, 1331, 444

Numbers like 123 or -121 ❌ are not palindromes.

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🧩 Problem Statement

👉 Write a function that checks whether a number is a palindrome.

• Input: 121

• Output: true ✅ (because reading it backwards still gives 121)

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🖥️ Solution Approach

We’ll solve it using math (without converting to string) 🚀

Steps:

1. If the number is negative → directly return false (since -121 ≠ 121-).

2. Store a copy of the number (xCopy).

3. Reverse the digits of the number.

   • Use % 10 to extract last digit.

   • Multiply the reversed number by 10 and add the last digit.

   • Use Math.floor(n / 10) to remove the last digit.

4. Compare reversed number with original copy.

   • If equal → it’s a palindrome.

   • Else → not a palindrome.

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🧑‍💻 Code Implementation

function isPalindrome(x) {
    // Step 1: Handle negatives
    if (x < 0) return false;

    // Step 2: Copy number
    let xCopy = x;
    let rev = 0;

    // Step 3: Reverse digits
    while (x > 0) {
        let rem = x % 10;             // last digit
        rev = (rev * 10) + rem;       // add to reversed
        x = Math.floor(x / 10);       // remove last digit
    }

    // Step 4: Compare
    return rev === xCopy;
}

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🔍 Dry Run Example

👉 Input: 121

Finally → rev(121) === xCopy(121) ✅ Palindrome!

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🎯 Test Cases

console.log(isPalindrome(121));   // true
console.log(isPalindrome(-121));  // false (negative numbers not palindromes)
console.log(isPalindrome(10));    // false (reverse = 01 → 1)
console.log(isPalindrome(1331));  // true
console.log(isPalindrome(123));   // false

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🛠️ Alternate Approaches

1. Convert to String

    function isPalindromeStr(x) {
      let str = x.toString();
      return str === str.split("").reverse().join("");
    }
   

   ✅ Short and simple, but less efficient (extra memory used).

2. Two-pointer technique on string
   Compare first and last characters, then move inward.

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📌 Key Takeaways

• Palindrome check is a common coding interview problem.

• Avoiding string conversion shows stronger algorithmic skills.

• Practice variations:

  • Palindrome strings

  • Palindrome with spaces/ignoring cases ("A man a plan a canal Panama")

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✨ You now have a clear understanding of how to check palindrome numbers in JavaScript — both mathematically and using strings.