202.md

202. Happy Number

难度: Easy

刷题内容

原题连接

• https://leetcode.com/problems/happy-number/

内容描述

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

解题方案

思路 1 - 时间复杂度: O(1) - 空间复杂度: O(1)

需要注意的是避免死循环

class Solution:
    def isHappy(self, n):
        """
        :type n: int
        :rtype: bool
        """
        def squareSum(num):
            res = 0
            for i in str(num):
                res += int(i) ** 2
            return res
                
        lookup = set()
        sum = squareSum(n)
        while sum not in lookup and sum != 1:
            lookup.add(sum)
            sum = squareSum(sum)
        return True if sum == 1 else False

可以看看这两篇post

1. All you need to know about testing happy number!
2. c++ using Floyd Cycle Detection Algorithm

会有一个更深的认知