> For the complete documentation index, see [llms.txt](https://dfine.gitbook.io/leetcode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://dfine.gitbook.io/leetcode/202.happy_number.md).

# 202.Happy Number

**202.Happy Number**

难度:Easy

> 编写一个算法来判断一个数是不是“快乐数”。

一个“快乐数”定义为：对于一个正整数，每一次将该数替换为它每个位置上的数字的平方和，然后重复这个过程直到这个数变为 1，也可能是无限循环但始终变不到 1。如果可以变为 1，那么这个数就是快乐数。

示例:

```
输入: 19
输出: true
解释: 
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1
```

按题意来，

```
class Solution {
public:
    bool isHappy(int n) {
        if(n ==1 ) return true;
        if(n<4)
            return false;
        string s=to_string(n);
        unordered_set<int> before;
        while(true)
        {
        int res=0;
        for(auto c:s)
            res+=pow(c-'0', 2);
        s=to_string(res);
          //  cout<<res<<endl;
       
        if (res==1) return true;      
        if(before.count(res)) break;
        else        before.insert(res);
        
        }
        return false;
    }
};
```


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