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.
思路：
按照题目要求写一个函数不断求出一个数字的next，判断next是否等于1，不等于1就循环或递归继续求下去。
坑：如果这个数字不是happy num，那么它的next会是一个死循环，因此需要用一个hashset来记录已经生成过的数字。
public boolean isHappy(int n) {
HashSet<Integer> set = new HashSet<>();
while (set.add(n)) {
int res = 0, tmp = 0;
while (n > 0) {
tmp = n % 10;
res += tmp * tmp;
n /= 10;
}
if (res == 1) {
return true;
} else {
n = res;
}
}
return false;
}
