JavaScript Program to Check for Palindrome String using Recursion

Given a string, write a recursive function that checks if the given string is a palindrome, else, not a palindrome. A string is called a palindrome if the reverse of the string is the same as the original one. For example – “madam”, “racecar”, etc.

What is Recursion?

The process in which a function calls itself directly or indirectly is called recursion and the corresponding function is called a recursive function. In the recursive program, the solution to the base case is provided and the solution to the bigger problem is expressed in terms of smaller problems. 

Examples:

Input : NITIN
Output : Yes
Reverse of NITIN is also NITIN.
Input : CAR
Output : No
Reverse of CAR is not CAR it is RAC

Approach 1:

• If there is only one character in the string, return true.
• Else compare the first and last characters and recuring for the remaining substring.

Example:

Javascript

// A recursive JavaScript program to
// check whether a given String
// is palindrome or not

function checkPalindrome(str, s, e) {

    // If there is only one character
    if (s == e)
        return true;

    // If first and last characters
    // does not match
    if ((str.charAt(s)) != (str.charAt(e)))
        return false;

    // If there are more than
    // two characters, check if
    // middle substring is also
    // palindrome or not.
    if (s < e + 1)
        return checkPalindrome(str, s + 1, e - 1);

    return true;
}

function isPalindrome(str) {
    let len = str.length;

    // An empty string is
    // considered as palindrome
    if (len == 0)
        return true;

    return checkPalindrome(str, 0, len - 1);
}

// Driver Code

let str = "malayalam";

if (isPalindrome(str))
   console.log("Yes, it is palindrome");
else
    console.log("No,it is not palindrome");

Output

Yes, it is palindrome

Approach 2:

• In this approach, we will check while traversing whether the ith and n-i-1th index are equal or not.
• If there are not equal return false and if they are equal then continue with the recursion calls.

Example:

Javascript

function checkPalindrome(s, i) {
    if (i > s.length / 2) { return true; }
    return s[i] == s[s.length - i - 1] && checkPalindrome(s, i + 1)
}

let str = "racecar";
let ans = checkPalindrome(str, 0);
if (ans == true) {
    console.log("Yes,it is palindrome");
}
else {
    console.log("No,it is not palindrome");
}

Output

Yes,it is palindrome

Time Complexity: O(n)

Auxiliary Space: O(n)