202. 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

  • 12 + 92 = 82
  • 82 + 22 = 68
  • 62 + 82 = 100
  • 12 + 02 + 02 = 1
class Solution {
    public boolean isHappy(int n) {
        HashSet set = new HashSet();
        int re;
        while(set.add(n)){
            int sum =0;
            while(n>0){
                re = n%10;
                sum += re * re;
                n /= 10;
            }
            if(sum ==1) return true;
            n = sum;
        }
        return n==1;
    }
}

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