LeetCode (24) Happy Number

题目描述

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

• $1^2 + 9^2 = 82$
• $8^2 + 2^2 = 68$
• $6^2 + 8^2 = 100$

本题比较简单，题目要求对任意一个正整数，不断各个数位上数字的平方和，若最终收敛为1，则该数字为happy number，否则程序可能从某个数开始陷入循环。这道题目我们只用根据规则进行计算，并使用一个哈希表存储已经出现过的数字即可。

class Solution {
public:
    bool isHappy(int n) {
        if (n < 1)
            return false;
        if (n == 1)
            return true;
        unordered_set<int> showedNums;
        showedNums.insert(n);

        while(true)
        {
            int s = 0;
            while(n)
            {
                s += (n % 10) * (n % 10);
                n = n / 10;
            }

            if (s == 1)
                return true;
            else if (showedNums.find(s) != showedNums.end())
                return false;
            n = s;
            showedNums.insert(s);
        }
    }
};