missingNumber.js

/**
 * 
268. Missing Number

Given an array nums containing n distinct numbers in the range [0, n], return the only number in the range that is missing from the array.

Follow up: Could you implement a solution using only O(1) extra space complexity and O(n) runtime complexity? 

Example 1:

Input: nums = [3,0,1]
Output: 2
Explanation: n = 3 since there are 3 numbers, so all numbers are in the range [0,3]. 2 is the missing number in the range since it does not appear in nums.
*/

/**
 * @param {number[]} nums
 * @return {number}
 */
var missingNumber = function (nums) {
  return nums.reduce((sum, cur, index) => (sum += index + 1 - cur), 0);
};