问题
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/
回答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.)
来源：https://stackoverflow.com/questions/50749527/why-does-my-python-recursive-function-return-none-instead-of-a-boolean