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

• 12 + 92 = 82
• 82 + 22 = 68
• 62 + 82 = 100
• 12 + 02 + 02 = 1

本题比较简单，做一个hashset来记录是否有重复的数字出现过，如果没有出现过，就把这个数加进去，然后求各位数平方和，看是否为1，不为1的话，就继续循环。代码如下：

public class Solution {
    public boolean isHappy(int n) {
        Set<Integer> set = new HashSet<Integer>();
        if(n==1) return true;
        while(!set.contains(n)){
            set.add(n);
            int sum =0;
            while(n!=0){
                int s = n%10;
                sum+=s*s;
                n = n/10;
            }
            if(sum==1) return true;
            n = sum;
        }
        return false;
    }
}