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

1^2 +9^2 = 82

8^2 + 2^2 = 68

6^2 + 8^2 = 100

1^2 + 0^2 + 0^2 = 1

关键点：之前题也做过了，如何判断这个数

在进行操作后还是没变？我们可以利用Set集合来进行判断，代码如下：

public boolean isHappy(int n) {
Set<Integer> OutLoop = new HashSet<Integer>();
int sum = 0;//当前和
int rail_num = 0;//尾数
//关键是如何来停止这个 while循环呢？
//如果n不能再加入，那么就停止Loop

while(OutLoop.add(n)){
sum = 0;
while (n > 0) {
rail_num = n%10;
sum += rail_num*rail_num;
n /= 10;
}
if(sum==1) return true;
else        n = sum;
}
return false;

}