题目描述：
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
解题思路：
用Set记录平方和的值，如果该值出现过，则表明进入新的循环，返回false；否则继续计算平方和，直到和为1为止。
代码如下：
public class Solution {
public boolean isHappy(int n) {
Set<Integer> set = new HashSet<Integer>();
int sum = digitSquareSum(n);
while(sum != 1){
if(!set.add(sum))
return false;
sum = digitSquareSum(sum);
}
return true;
}
public int digitSquareSum(int n){
int res = 0;
while(n > 0){
res += (n % 10) * (n % 10);
n /= 10;
}
return res;
}
}