202. Happy Number

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本文介绍了一种用于判断任意正整数是否为快乐数的算法。快乐数是指通过反复替换该数为它每个位数平方的和，最终能够得到1的数字。文章详细解释了这一过程，并提供了使用Java实现的具体代码。

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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

Credits:
Special thanks to @mithmatt and @ts for adding this problem and creating all test cases.

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Solution:

Tips:

use set to record the N.

Java Code:

public class Solution {
    public boolean isHappy(int n) {
        Set<Integer> si = new HashSet<>();
        int currentN = n;
        while (true) {
            int sum = 0;
            while (currentN > 0) {
                int m = currentN % 10;
                sum += m * m;
                currentN /= 10;
            }
            currentN = sum;
            if (1 == sum) {
                break;
            }
            
            if (si.contains(sum)) {
                return false;
            }
            
            si.add(sum);
        }
        
        return true;
    }
}