Loops vs Recursions

Published on Aug 10, 2020

In high level languages, like JavaScript, there are handy methods that allow us to get to the point in a functional way.

For example, to know if a string is palindrome (that reads the same backwards) we can do:

function isPalindrome(string) {
    return string.split('').reverse().join('') == string

Anyway, not all languages provide us easy methods like the ones above.

In such cases, we can approach the solution to the problem in the good old way: using loops or recursions.


function isPalindrome(string) {
    for (let l = 0; l < string.length; l++) {
        let r = string.length - (l+1)
        if (string[l] != string[r]) return false
    return true

We set a left pointer l starting at the beginning of the string and a right pointer r starting at the end of the string.

At every loop we move left pointer to the right and right pointer to the left.

If the two characters at the two given positions are different, we break the loop, returning false.

If the loop exits, it means that the string was traversed completely and the characters were always corresponding.


function isPalindrome(string, l) {
    let r = string.length - (l+1)
    if (l == string.length) return true
    if (string[l] != string[r]) return false
    return isPalindrome(string, l+1)

Recursions are a little trickier to write. Anyway, there are 3 tips to keep in mind to master them.

When writing recursion we need:

  1. A variable to pass current state to children calls
  2. A final condition that can be returned up to the parent call
  3. A recursive call if the final condition is not met

Given that in recursions there is no way for the function to know current state, we pass it on first call (left pointer l = 0).

At every recursion call we check if at the given positions (l and r):

If none of the final conditions are met - we keep running - updating the current state (left pointer l = l+1) and calling the next recursion.

We pass only left l pointer, as right r position is easily derivable from the first one. As for the loop solution we are moving left pointer to the right and right pointer to the left until the end of the string.

Big O notation

In both cases the execution time is O(N), fair enough!

Written by

Simone Manzi


Senior Software Engineer with strong expertise in Web Development, now writing bugs as Data Engineer.
Thinks of himself to be a real Full-Stack... Only until his impostor syndrome does not take over!