Happy Number

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

 

C解法

bool isHappy(int n) {
    int lastOne = 0, nextNum = 0;
    if(n == 1)
        return true;
    else if(n == 0 || n == 2 || n == 3 || n == 4)    //之所以把4放进去是因为后来发现所有的回归都是回归到4的
        return false;
    else
    {
        while(n)
            {
                lastOne = n % 10;
                nextNum += lastOne * lastOne;
                n = n / 10;
            }
        
        return isHappy(nextNum);            //树的后遗症，一直递归
}

C++解法：

class Solution {
public:
    bool isHappy(int n) {
        int lastNum = 0, getNum = 0;
        if(n == 1)
            return true;
        else if(n == 0 || n == 2 || n == 3)
            return false;            
        else
        {
            unordered_set<int> ln;            //建立一个set来保存遇到过的值
            while(true)
            { 
                ln.insert(n);
                getNum = 0;
                while(n > 0)
                {
                    lastNum = n % 10;
                    getNum += lastNum * lastNum;
                    n /= 10;
                }
                if (getNum == 1)
                    return true;
                else if (ln.find(getNum) != ln.end())    //如果在set中发现之前出现过，说明循环了，返回0
                    return false;
                n = getNum;
            }
        }
    }
};

• 这是一道哈希表的题；用C自己再写一个哈希表，太费力了，直接使用C++的库
• unordered_set：就是用哈希形式建立的无序容器