Happy_Number.cpp

// Source : https://leetcode-cn.com/problems/happy-number/description/
// Number : 202
// Author : HL
// Date   : 2018-09-11
// Kill   : 100.00%，100.00%

/********************************************************************************** 
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
**********************************************************************************/

class Solution {
public:
    bool isHappy(int n) {
        if(n == 1)
            return true;
        // 循环解法（不严谨，但可以通过测试）
        // int count = 0;
        // while(count<10)
        // {
        //     vector<int> v;
        //     int sum = 0;
        //     while(n>0)
        //     {
        //         v.push_back(n%10);
        //         n /= 10;
        //     }
        //     for(vector<int>::iterator it=v.begin(); it!=v.end(); it++)
        //         sum += (*it)*(*it);
        //     if(sum==1)
        //         return true;
        //     else
        //         n = sum;
        //     count++;
        // }
        // return false;
        
        // 集合解法
        set<int> s;
        while(s.find(n)==s.end())
        {
            s.insert(n);
            int sum = 0;
            while(n>0)
            {
                sum += (n%10)*(n%10);
                n /= 10;
            }
            if(sum==1)
                return true;
            else
                n = sum;
        }
        return false;
    }
};