202. Happy Number

1. 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: 19 is a happy number
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

Tags: Hash Table Math
Similar Problems: (E) Add Digits (E) Ugly Number

2. 解题思路

• 利用集合，判断是否陷入循环！！！

3. 代码

class Solution {
public:
    bool isHappy(int n) 
    {
        std::set<int> tSet;
        tSet.insert(n);
        while (true)
        {
            int t = Sum(n);
            if (t == 1)
            {
                return true;
            }
            if (0 != tSet.count(t))
            {
                return false;
            }
            tSet.insert(t);
            n = t;
        }
    }
    int Sum(int n)
    {
        int sum = 0;
        while (0 != n)
        {
            int t = n%10;
            n = n/10;
            sum += t*t;
        }
        return sum;
    }
};

4. 反思