问题描述

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

参考答案

class Solution {
public:
    bool isHappy(int n) {
        unordered_set<int> temp;
        while(n != 1){
    
            if(temp.find(n) == temp.end())
                temp.insert(n);
            else
                return false;
            
            int sum = 0;
            while(n!=0){
                sum += pow(n%10,2);
                n = n/10;
            }

            n = sum;
        }
        return true;
        
    }
};

答案分析

答案分成两部分：扔进hashset里，计算结果。

第一部分。把每次的n都加入hashset中，使用 hashset.end() 判断是否为空。如果重复了，那么说明算了一圈算回来了，不是 happy number。

第二部分。和例子一样，每次sum=0，然后 update the value of n. 

最后 n 成为了 1， 说明是 happy number。

LC 202. Happy Number