[leetcode] 202. Happy Number

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本文介绍了一种用于判断正整数是否为快乐数的算法。快乐数是指通过将该数替换为其各位数字平方之和并重复此过程直到数字变为1的过程中的数。如果这个过程最终导致1，则称该数为快乐数；如果进入一个不包含1的循环，则不是快乐数。文章提供了一个使用C++实现的具体示例。

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

Solution:

Idea: For happy numbers, the number will equal 1 at some point. For others, the numbers will show in cycles. For instance,

<span style="font-size:14px;">class Solution {
public:
    bool isHappy(int n) {
        unordered_map<int,int> hs;
        bool ish = false;
        while(1){
            n = digit_square_sum(n);
            if (n==1){
                ish = true;
                break;
            }
            else{
                hs[n]++;
                if(hs[n]>1){
                    ish = false;
                    break;
                }
            }
        }
        return ish;
    }
    int digit_square_sum(int n){
        int sum = 0;
        while(n){
            sum += pow(n%10,2);
            n = n/10;
        }
        return sum;
        
    }
};</span>