202. 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

 

1^2 + 9^2 = 82

8^2 + 2^2 = 68

6^2 + 8^2 = 100

1^2 + 0^2 + 0^2 = 1

 

解题思路：

方法一：使用set保存中间值，每次判断中间结果是否在set中，或者判断中间结果是否为1.。

方法二：利用Happy Number的一个性质。。https://en.wikipedia.org/wiki/Happy_number

// using set costs 4ms class Solution { public:     bool isHappy(int n) {        set<int>s;        if(n==0)return false;        s.insert(n);        while(true){            int tmp=0;            while(n){                tmp+=(n%10)*(n%10);                n/=10;            }            n=tmp;            if(n==1)return true;            if(s.find(n)!=s.end())return false;            s.insert(n);        }        return false;     } };   // https://en.wikipedia.org/wiki/Happy_number // Happy_number has attribute all happy number ends in 1,all non-happy number ends in 4; // costs 0ms sometimes class Solution { public:     bool isHappy(int n) {        if(n==0)return false;        int tmp=0;        while(n!=1&&n!=4){            while(n){                tmp+=(n%10)*(n%10);                n/=10;            }            n=tmp;            tmp=0;        }        return n==1;     } };