Palindrome Check - JavaScript Program

// Palindrome: reads same forwards and backwards // Examples: 121, "madam", "racecar" // Method 1: String palindrome function isPalindromeString(str) { str = str.toLowerCase().replace(/[^a-z0-9]/g, ''); let reversed = str.split('').reverse().join(''); return str === reversed; } console.log("madam:", isPalindromeString("madam")); console.log("hello:", isPalindromeString("hello")); console.log("A man a plan a canal Panama:", isPalindromeString("A man a plan a canal Panama")); // Method 2: Number palindrome function isPalindromeNumber(num) { let original = num; let reversed = 0; while (num > 0) { reversed = reversed * 10 + (num % 10); num = Math.floor(num / 10); } return original === reversed; } console.log("\nNumber palindrome:"); console.log("121:", isPalindromeNumber(121)); console.log("123:", isPalindromeNumber(123)); console.log("1221:", isPalindromeNumber(1221)); // Method 3: Two-pointer approach (efficient) function isPalindromeTwoPointer(str) { str = str.toLowerCase().replace(/[^a-z0-9]/g, ''); let left = 0; let right = str.length - 1; while (left < right) { if (str[left] !== str[right]) { return false; } left++; right--; } return true; } console.log("\nTwo-pointer approach:"); console.log("racecar:", isPalindromeTwoPointer("racecar")); console.log("hello:", isPalindromeTwoPointer("hello")); // Method 4: Recursive approach function isPalindromeRecursive(str) { str = str.toLowerCase().replace(/[^a-z0-9]/g, ''); if (str.length <= 1) return true; if (str[0] !== str[str.length - 1]) return false; return isPalindromeRecursive(str.slice(1, -1)); } console.log("\nRecursive approach:"); console.log("level:", isPalindromeRecursive("level")); console.log("world:", isPalindromeRecursive("world"));

madam: true hello: false A man a plan a canal Panama: true Number palindrome: 121: true 123: false 1221: true Two-pointer approach: racecar: true hello: false Recursive approach: level: true world: false

This program demonstrates different methods to check if a value is a palindrome.

Palindrome Definition

A palindrome reads the same forwards and backwards:

Numbers: 121, 1221, 1331
Strings: "madam", "racecar", "level"
Phrases: "A man a plan a canal Panama"

Method 1: String Reversal

Compare original with reversed:

javascript
let reversed = str.split('').reverse().join('');
return str === reversed;

String Cleaning:

toLowerCase(): Case-insensitive
replace(/[^a-z0-9]/g, ''): Remove non-alphanumeric

Method 2: Number Palindrome

Reverse number and compare:

javascript
let reversed = 0;
while (num > 0) {
    reversed = reversed * 10 + (num % 10);
    num = Math.floor(num / 10);
}
return original === reversed;

Method 3: Two-Pointer Approach

Compare characters from both ends:

javascript
let left = 0;
let right = str.length - 1;
while (left < right) {
    if (str[left] !== str[right]) return false;
    left++;
    right--;
}
return true;

Pros:

O(n) time, O(1) space
No string reversal needed
Most efficient

Method 4: Recursive

Check first and last, recurse on middle:

javascript
if (str[0] !== str[str.length - 1]) return false;
return isPalindromeRecursive(str.slice(1, -1));

String Methods:

slice(1, -1): Remove first and last character
charAt(0): Get first character
length: String length

Time Complexity:

Reversal: O(n)
Two-pointer: O(n) - Best!
Recursive: O(n)

When to Use:

Reversal: Simple, readable
Two-pointer: Most efficient
Recursive: Learning recursion

Output