```
//https://www.kidscodecs.com/happy-numbers/
int isHappy(int n)
{
int sum = 0;
for(; n; n/=10)
sum += (n%10)*(n%10);
if(sum == 1)
return true;
int arr[] = {4, 16, 37, 58, 89, 145, 42, 20};
for(int i = 0; i < sizeof(arr)/sizeof(int); i++)
if(sum == arr[i])
return false;
return isHappy(sum);
}
```

```
int squareSum(int n)
{
int sum=0, t;
while(n)
{
t = n%10;
sum += t*t;
n /= 10;
}
return sum;
}
//AC - 0ms - there must be a loop here - since the squareSum is finite while the loop is infinite;
int isHappy(int n)
{
int slow=n, fast=n;
do
{
slow = squareSum(slow);
fast = squareSum(fast);
fast = squareSum(fast);
} while(slow != fast);
if(slow == 1) return 1;
else return 0;
}
```

```
int squareSum(int n)
{
int sum=0, t;
while(n)
{
t = n%10;
sum += t*t;
n /= 10;
}
return sum;
}
int isHappy(int n)
{
int slow=n, fast=n;
while(slow > 1)
{
slow = squareSum(slow);
if(slow == 1) return true;
fast = squareSum(squareSum(fast));
if(fast == 1) return true;
if(slow == fast) return false;
}
return true;
}
```

```
//only 1 and 7 in [0, 9] is happy number, and since there must a loop and each number will be equivalent to [0, 9];
int isHappy(int n)
{
while(n > 6)
{
int next = 0;
while(n)
{
next += (n%10)*(n%10);
n /= 10;
}
n = next;
}
return n == 1;
}
```