Happy Number

package cn.edu.xidian.sselab.hashtable;

import java.util.HashSet;
import java.util.Set;

/**
 *
 * @author zhiyong wang
 * title： Happy Number
 * content:
 * 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
 *
 *
 */
public class HappyNumber {

    //自己改了三次才改正确的方法,出现了两个错误：（1）忘记如果不能于1，会一直循环的情况给排除掉,这里用一个Set保存出现的值来排除这种情况
    //（2）求平方的时候没有用Math.pow(a,b)这种方法
    public boolean isHappy(int n){
        if(n < 1)
            return false;
        Set set = new HashSet();
        while(n > 1){
            if(!set.add(n))
                return false;
            n = calculate(n);
        }
        if(n == 1)
            return true;
        else
            return false;
    }
    
    private int calculate(int n){
        int result = 0;
        while(n > 0){
            result += Math.pow(n%10, 2);
            n = n/10;
        }
        return result;
    }

}

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