In this post I'll be walking through the Happy Number Algorithm, a problem recently featured on Leetcode's 30 Day Challenge in April (you can find the problem here).

Here's the question:

Write an algorithm to determine if a number n is "happy".

A happy number is a number defined by the following process: Starting with any positive integer, replace the number by the sum of the squares of its digits, and repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1. Those numbers for which this process ends in 1 are happy numbers.

Return True if n is a happy number, and False if not.

For example, let's say your input was 19. The expected output would be `true`

. 19, when broken down into its digits, is 1 and 9. Those numbers squared are 1 and 81. 1 + 81 is 82. 82 broken into its digits is 8 and 2. Those numbers squared are 64 and 4, and 64 + 4 = 68. 68 broken down into its digits is 6 and 8. Those numbers squared is 36 and 64. 36 + 64 = 100. 100 broken down into its digits is 1, 0, and 0. Those numbers squared and summed is 1, which makes it a happy number.

There are a number of ways to approach this problem, and in this post I'll be walking through how to use recursion to solve it.

The first thing I'll do is initialize a variable called sum. The digits of a number will be broken up, set to the power of two, and then added, so it's important to keep track of what the sum is.

```
function isHappy(n) {
let sum = 0;
//...
}
```

Then, I'll want to break up a number using modulo. As I've explained in a previous blog post, when asked to manipulate a number, it's best to do so without changing it to a string or integer. Modulo enables you do to this easily. If given n = 25, n%10 would give you the result of 5, and n would be 20. Then, we'll want to divide n by 10 in order to shift the digits over. We'll keep doing this as long as n is greater than 0. The other thing to do in this while loop is to add the square of each result of the modulo to the sum. Written out, the function now looks like this:

```
function isHappy(n) {
let sum = 0;
while (n > 0) {
let e = n % 10;
n = Math.floor(n / 10);
sum += e * e;
}
//...
}
```

Now, we need to check what the sum is. If the sum is equal to 1, then it's a happy number, and we can return true.

```
function isHappy(n) {
let sum = 0;
while (n > 0) {
let e = n % 10;
n = Math.floor(n / 10);
sum += e * e;
}
if (sum === 1) {
return true;
}
//...
```

If the sum is greater than 1 but less than or equal to 4, then we know the sum will get stuck in an infinite loop and will never equal 1, so it's definitely not a happy number. You can test this out yourself: if the number is 2, then 2^2 is 4. 4^2 is 16. 1^2 + 6^2 = 37. 3^2 + 7^2 = 58. 5 ^2 + 8^2 = 89. 8^2 + 9^2 = 145. 1^2 + 4^2 + 5+2 = 42. 4^2 + 2^2 = 20. 2^2 + 0^2 = 4 -- and we're stuck in the same loop. (You can also try this out for 3). Therefore, if the sum is greater than 1 and less than or equal to 4, then we can return false.

```
function isHappy(n) {
let sum = 0;
while (n > 0) {
let e = n % 10;
n = Math.floor(n / 10);
sum += e * e;
}
if (sum === 1) {
return true;
} else if (sum > 1 && sum <= 4) {
return false;
}
//...
}
```

The final thing to do is make a recursive call to the function. If the sum does not fulfill one of the base cases--it's not equal to 1, 2, 3, or 4--then its digits need to be split, squared, and summed, therefore calling the function again. Therefore, we need to call the function, this time with `sum`

as the argument. It's also important to write 'return' before the function call, or else you'll end up with a result of 'undefined', since nothing was returned back.

```
function isHappy(n) {
let sum = 0;
while (n > 0) {
let e = n % 10;
n = Math.floor(n / 10);
sum += e * e;
}
if (sum === 1) {
return true;
} else if (sum > 1 && sum <= 4) {
return false;
}
return isHappy(sum);
}
```

And that's it! Feel free to leave alternate solutions or questions in the comments.

## Discussion

Your solution makes sense, mathematically.

However, I think it's a wasted chance to use and explain recursion with carryover.

The function can be something like this:

You could use

`Set`

instead of an array for`seenSums`

, that way your validation to check the infinite loop could be done in constant time by changingfor

Why aren't you adding the square of n to the sum?

I'm dividing n into it's digits (so 19 becomes 1 and 9) and then adding the square of those numbers to the sum.

Yeah I got that, but I don't see sum += n*n anywhere

The line 'sum += e * e' is what sums the digits

Ahh, I missed the while loop there. Oops

Hi, why a number is not a happy number if the sum is greater than 1 but less than or equal to 4? Why not 5 or 6? Thank you.