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

分析：
这题的关键是如何退出循环如果那个数不是hapy number. 这里用了一个方法，就是用hashset保存得到过的数，如果出现重复，那就不是Happy number.

public class Solution {
    /**
     * @param n an integer
     * @return true if this is a happy number or false
     */
    public boolean isHappy(int n) {
        if (n < 1) return false;
        
        Set<Integer> hashSet = new HashSet<Integer>();
        hashSet.add(n);
        
        while (n != 1) {
            n = getNewNumber(n);
            if (hashSet.contains(n)) {
                return false;
            }
            hashSet.add(n);
        }
        return true;
    }
    
    public int getNewNumber(int n) {
        int total = 0;
        while (n != 0) {
            total += (n % 10) * (n % 10);
            n = n / 10;
        }
        return total;
    }
}

 

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

    boolean 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 true;
        return false;
    }
}