find the happy number 博客分类： 算法 javahappy numberalgorithm

this question is come from leetcode:

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

the value of this question: how to define it will loops endlessly? 100, 1000, 10000 or more times without end in 1? maybe all of them not right.
unless we find that there have a cycle, we make sure that there have a endless loop.
So we need to use two point, if the fast point = the slow point, then it approved that there have a cycle. this is a endless loop.

java solution as below:

 public boolean isHappy(int n) {
        if(n <= 0) return false;
        int times = 100;
        int slow = n;
        int fast = n;
        do{
            slow = nextNumber(slow);
            fast = nextNumber(fast);
            fast = nextNumber(fast);
            if(fast == 1) return true;
        }while(slow != fast);
        
        return false;
    }
    
    private int nextNumber(int n){
        int result = 0;
        while(n > 0){
            result += Math.pow(n%10, 2);
            n = n/10;
        }
        return result;
    }