Why does my python recursive function return None instead of a boolean?

Question

The Question is:

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: 
1^2 + 9^2 = 82
8^2 + 2^2 = 68
6^2 + 8^2 = 100
1^2 + 0^2 + 0^2 = 1

My Code is as follows:

class Solution:
    def isHappy(self, n):

        x = sum(list(map(lambda a: int(a)**2, list(str(n)))))
        if x == 1:
            return True

        self.isHappy(x)

But it doesnt work with an input of 19. The program terminates at x == 1 meaning that it works with the if condition. But it keeps saying that it returns "Null" instead of True. Why is this? I am assuming I am missing something with recursion...?

Source for question: https://leetcode.com/problems/happy-number/description/

your need to return the `self.isHappy(x)` else it won't go back to the top of the stack — Liora Haydont, Jun 07 '18 at 20:16
Aside from that, you're also not handling the infinite-cycle-not-involving-1 case. — user2357112, Jun 07 '18 at 20:17

score 1 · Accepted Answer · answered Jun 07 '18 at 20:20

1

You don't return anything when the first iteration did not give 1 as sum. Change the last line into

return self.isHappy(x)

(Then you'll find your solution enters an infinite loop when presented with an unhappy number.)

answered Jun 07 '18 at 20:20

bipll

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Why does my python recursive function return None instead of a boolean?

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