Input: [3,0,1]
Output: 2

Input: [9,6,4,2,3,5,7,0,1]
Output: 8

/**
 * @param {number[]} nums
 * @return {number}
 */
var missingNumber = function(nums) {
    if (nums.length === 0) return -1;
    let sumOfNums = nums.reduce((ac, x) => ac + x);
    let sumTillN = (nums.length * (nums.length + 1)) / 2;
    return sumTillN - sumOfNums;
};