find-all-duplicates-in-an-array.js

/**
 * Find All Duplicates in an Array
 *
 * Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.
 *
 * Find all the elements that appear twice in this array.
 *
 * Could you do it without extra space and in O(n) runtime?
 *
 * Example:
 *
 * Input:
 * [4,3,2,7,8,2,3,1]
 *
 * Output:
 * [2,3]
 */

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

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

    if (nums[index] < 0) {
      result.push(Math.abs(nums[i]));
    } else {
      nums[index] = -nums[index];
    }
  }

  return result;
};

export { findDuplicates };