find-all-numbers-disappeared-in-an-array.js

/**
 * Find All Numbers Disappeared in an Array
 *
 * Given an array of integers where 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.
 *
 * Find all the elements of [1, n] inclusive that do not appear in this array.
 *
 * Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count as extra space.
 *
 * Example:
 *
 * Input:
 * [4,3,2,7,8,2,3,1]
 *
 * Output:
 * [5,6]
 */

/**
 * @param {number[]} nums
 * @return {number[]}
 */
const findDisappearedNumbers = nums => {
  const result = [];

  for (let i = 0; i < nums.length; i++) {
    const index = Math.abs(nums[i]) - 1;
    if (nums[index] > 0) {
      nums[index] = -nums[index];
    }
  }

  for (let i = 0; i < nums.length; i++) {
    if (nums[i] > 0) {
      result.push(i + 1);
    }
  }

  return result;
};