Contest - Day 2 - Happy Number

Leetcode is running a 30 day “Covid 19” style contest for everyone on self quarantine and social distancing. Let’s jump in!

Happy Number

Problem

Write an algorithm to determine if a number 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.

Example:

Input: 19
Output: true
Explanation: 
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

Thinking

Once you know how to pull individual digits out of a number, these are relatively straight forward. You just need to keep track of your in-progress sums vs. total sums and the total sums you’ve previously seen for loop detection.

To get a digit, you just mod the number by its base (10, assuming its base 10). Then you divide the original number by its base and continue. An unrolled look for the number 19 would look like:

x = 19
x % 10  # => 9
x //= 10  # (x => 1)
x % 10  # => 1
x //= 10  # (x => 0)

Corner Cases

I don’t think I missed any this time.

Improvements

There’s possibly a way to detect loops without using a set (mathematical) to decrease memory usage but I don’t know of it.

Solution

class Solution:
    def isHappy(self, n: int) -> bool:
        seen = set()
        curr = n
        prev = n

        while prev != 1:
            if prev in seen:
                break

            seen.add(prev)
            curr = prev
            prev = 0   

            while curr:
                digit = curr % 10
                curr //= 10
                prev += digit ** 2

        return prev == 1

Score

Runtime: 36 ms, faster than 30.40% of Python3 online submissions for Happy Number.
Memory Usage: 13.9 MB, less than 5.26% of Python3 online submissions for Happy Number.

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