LC 202 - Happy Number

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.
• 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.

Return true if n is a happy number, and false if not.

Example 1:

Input: n = 19
Output: true

Explanation: 12 + 92 = 82 82 + 22 = 68 62 + 82 = 100 12 + 02 + 02 = 1

Example 2:

Input: n = 2
Output: false

Constraints:

• 1 <= n <= 231 - 1

Links

Question here and solution here

Solution

concept

We can check if a processed number is repeated, this will cause endless loop.

code

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

        def process(num: int) -> int:
            num_list = [int(i) for i in str(num)]
            ans = 0
            for n in num_list:
                ans += n**2
            return ans

        while True:
            n = process(n)
            if n in num_set:
                return False
            else:
                num_set.add(n)
            if n == 1:
                break
        
        return True
        
class NeetSolution:
    def isHappy(self, n: int) -> bool:
        visit = set()

        while n not in visit:
            visit.add(n)
            n = self.sumOfSquares(n)
            if n == 1:
                return True
        return False

    def sumOfSquares(self, n: int) -> int:
        output = 0

        while n:
            digit = n % 10
            digit = digit ** 2
            output += digit
            n = n // 10
        return output

Complexity

time: $O(\log n)$
space: $O(\log n)$