Happy Number

Question

Problem Statement

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

• $1^2 + 9^2 = 82$
• $8^2 + 2^2 = 68$
• $6^2 + 8^2 = 100$
• $1^2 + 0^2 + 0^2 = 1$

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

题解

Java

public class Solution {
public boolean isHappy(int n) {
if (n < 0) return false;

Set<Integer> set = new HashSet<Integer>();
while (n != 1) {
n = digitsSquareSum(n);
if (n == 1) {
return true;
} else if (set.contains(n)) {
return false;
} else {
}
}

return true;
}

private int digitsSquareSum(int n) {
int sum = 0;
for (; n > 0; n /= 10) {
sum += (n % 10) * (n % 10);
}
return sum;
}
}