Happy number check

Determine whether a number is happy by iterating the sum of squares of its digits.

math loops set numbers

A happy number is a number that eventually reaches 1 when replaced by the sum of the square of each digit repeatedly. If the process enters a cycle that does not include 1, the number is unhappy.

Write a function `is_happy(n: int) -> bool` that takes a positive integer `n` and returns `True` if `n` is happy, `False` otherwise.

For example, 19 is happy because:
- 1^2 + 9^2 = 82
- 8^2 + 2^2 = 68
- 6^2 + 8^2 = 100
- 1^2 + 0^2 + 0^2 = 1

2 is unhappy because it enters the cycle 4 → 16 → 37 → 58 → 89 → 145 → 42 → 20 → 4.

Constraints

- `n` is a positive integer (1 ≤ n ≤ 10^9)
- The function must return a boolean.
- Time complexity: O(number of iterations), which is small.

Example

>>> is_happy(19)
True
>>> is_happy(2)
False
>>> is_happy(1)
True
>>> is_happy(7)
True

8 points ~12 min

Recent Submissions

No submissions yet — hit Run Tests to try!

Hints

Use a set to remember numbers you have already seen.

While n is not 1 and not in the seen set, add n to the set and replace n with the sum of the squares of its digits.

If you reach 1, return True; if you encounter a number already seen, return False.

Use a helper function to compute the next number from a given number.

Test Results

Press Ctrl+Enter or click Run Tests to execute your code.