add-digits.js

// Given a non-negative integer num, repeatedly add all its digits until the result has only one digit. 
//
//
//
// For example:
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// Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.
//
//
// Follow up:
// Could you do it without any loop/recursion in O(1) runtime?
//
//
//
//   A naive implementation of the above process is trivial. Could you come up with other methods? 
//   What are all the possible results?
//   How do they occur, periodically or randomly?
//   You may find this Wikipedia article useful.
//
//
// Credits:Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.


/**
 * @param {number} num
 * @return {number}
 */
var addDigits = function(num) {
    var str = num.toString();
    if (str.length === 1) {
        return +str;
    }
    var result = str.split('').reduce((memo, current) => +memo + +current);
    return addDigits(result);
};