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

分析：如果算得数字为1,则为Happy Number；如果进入无限循环，则不是Happy Number。

采用set进行存储，每进行一次平方求和（假设结果为s），对set内的数字进行判断：如果set中已经出现s，则表示会进入循环，返回false；否则，将s加入set。

运行时间：7ms

class Solution {
public:
    bool isHappy(int n){
        set<int> s;
        int recur = compute(n);
        while(true){
            if(recur == 1) return true;
            else{
                if(s.find(recur) != s.end()) return false;
                else{
                    s.insert(recur);
                    recur = compute(recur);
                }
            }
        }
    }
    int compute(int n){
        int sum = 0;
        while(n){
            int temp = n % 10;
            sum += temp * temp;
            n /= 10;
        }
        return sum;
    }
};

Runtime: 4ms

class Solution {
private:
    int compute(int n){
        if(n == 0) return n;
        
        int result = 0;
        while(n){
            int temp = n % 10;
            result += temp * temp;
            n /= 10;
        }
        return result;
    }
public:
    bool isHappy(int n) {
        unordered_map<int, int> m;
        
        while(n != 1){
            n = compute(n);
            if(m.find(n) != m.end())
                return false;
            else
                m[n] = n;
        }
        return true;
    }
};