leetcode 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

这道题挺简单的，就按照它的逻辑写就好了。
public boolean isHappy(int n) {
HashSet<Integer> set=new HashSet<Integer>();
while(n!=1&&!set.contains(n)){
set.add(n);
int sum=0;
while(n>0){
int yu=n%10;
n=n/10;
sum+=yu*yu;
}
n=sum;
}
if(n==1){
return true;
}
else{
return false;
}
}大神表示：看到大多数人都使用hashset来记录值。而实际上，我们可以使用 Floyd Cycle detection 算法，使用在 Linked List Cycle 找环的入口的方法。既然之后有循环，我们可以看做后面有环。我们只需要看这个环的入口值是不是1。

int digitSquareSum(int n) {
int sum = 0, tmp;
while (n) {
tmp = n % 10;
sum += tmp * tmp;
n /= 10;
}
return sum;
}

bool isHappy(int n) {
int slow, fast;
slow = fast = n;
do {
slow = digitSquareSum(slow);
fast = digitSquareSum(fast);
fast = digitSquareSum(fast);
} while(slow != fast);
if (slow == 1) return 1;
else return 0;
}