This easy problem looks simple at first but it gets very tricky specially when finding out if your loop already started in an endless loop. I did try a couple of ideas without using a list to keep track of previous sums but all didn’t work. Eventually I ended up using a `mutableList`

to maintain a list of previous sums so I can look up and see if the new sum has been solved before. If it is, then it means we entered into an endless loop already.

```
class Solution {
val ssq: (Int) -> Int = {
var x = it
var sum = 0
while(x != 0){
sum += Math.pow((x % 10).toDouble(), 2.0).toInt()
x = x / 10
}
sum
}
fun isHappy(n: Int): Boolean {
var x = n
var start = 0
var prevs = mutableListOf<Int>()
while(x != 1){
x = ssq(x)
if(x in prevs) break
prevs.add(x)
}
return x == 1
}
}
```

```
Runtime: 140 ms, faster than 67.19% of Kotlin online submissions for Happy Number.
Memory Usage: 31.2 MB, less than 100.00% of Kotlin online submissions for Happy Number.
```