Happy Number

20200403
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:

Input: 19
Output: true
Explanation: 
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

Javascript:

/**
 * @param {number} n
 * @return {boolean}
 */
var isHappy = function(n, counter=0) {
    result = false;
    if(counter < 20){
        //切割n并算出各个位置上数字平方后存入array数组
        const array = n.toString().split("").map(n => n*n);
        //数组单位加和，参数：初始值，最终值，初始Index：0
        const sum = array.reduce((a, b)=> a + b, 0);
        
        if (sum ===1 ){
            result = true;
        } else {
            counter++;
            isHappy(sum, counter);
        }
        
        return result;
        
    }
};

sample 44 ms submission
Setを活用したバージョン

/**
 * @param {number} n
 * @return {boolean}
 */
var isHappy = function(n) {
  let seen = new Set();
  let current = n;
  
  while(true) {
    current = String(current).split('').reduce((acc, number) => (acc + Number(number)**2), 0)
    if(current === 1) {
      return true;
    } else if(seen.has(current)) {
      return false;
    } else {
      seen.add(current)
    }
  }
};