Problem：

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

Thought：

The idea is just simple, the hints of HashMap and Recursion will help you a lot.

Java Solution（ Wrong Answer）

public class Solution {
    private static Map<Integer, Integer> map = new
 
 HashMap<Integer, Integer>();
    public boolean isHappy(int n) {
        if (n == 1) {
            return true;
        } else if (map.containsKey(n)) {
            return false;
        } else {
            map.put(n, map.size());         
        }

        String str = n + "";
        char
 
[] c = str.toCharArray();
        int temp = 0;       
        for (int i = 0; i < c.length; i++) {
            temp += Integer.parseInt(c[i]+"") * Integer.parseInt(c[i]+"");
        }
        // recursion
        return isHappy(temp);
    }
}

Exception

But the problem is that the answer of LeetCode is different from that of local IDE, which means when I iuput 10 in my local IDE, the output is true. Details is in the picture below:

The solution is : try to declare the class variables as class instance variables instead of class staic variables; That’s because the judger runs all test cases in one go.

Java Solution（ Accepted）

public class Solution{
    // Don't declare the variable as a static one, or it will occur error when OJ;
    private Map<Integer, Integer> map = new HashMap<Integer, Integer>();

    public boolean isHappy(int n) {
        // print just for testing
        // System.out.println(n);

        if (n == 1) {
            return true;
        } else if (map.containsKey(n)) {
            return false;
        } else {
            map.put(n, map.size());         
        }

        String str = n + "";
        char[] c = str.toCharArray();
        int temp = 0;       
        for (int i = 0; i < c.length; i++) {
            temp += Integer.parseInt(c[i]+"") * Integer.parseInt(c[i]+"");
        }
        // recursion
        return isHappy(temp);
    }
}

If you have any questions or any bugs are found, please feel free to contact me.

Your comments and suggestions are welcome!