202. Happy Number

题目：

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

思路：
乍一看这个题目并不难，但是仔细想需要保证进行判断数字是否重复来保证不会进入死循环，因此想到Set结构，它的集合特性能够保证其中的元素不重复。

代码：

public class Solution {
public boolean isHappy(int n) {
if(n < 1){return false;}
//用set结构保证这个数字是否被计算过
HashSet<Integer> mySet = new HashSet<Integer>();
int t;
while(n != 1 && !mySet.contains(n)){
//先将数字加到集合中
mySet.add(n);
t = n;
n = 0;
while(t != 0){
n += (t%10)*(t%10);
t /= 10;
}
}
//若是因为数字被计算过那么说明这个数字不是happy number
if(n == 1){
return true;
}else{
return false;

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}
}
}