Finds duplicate elements in the array

/**
 * Finds elements occurring more than once in the given array and returns them in ascending order.
 * If no such element is found, returns [-1].
 * @param {number[]} a - The input array containing elements from 0 to N-1.
 * @param {number} n - The size of the array.
 * @returns {number[]} - An array containing elements occurring more than once in ascending order, or [-1] if none found.
 */
function duplicates(a, n) {
  // Sort the array in ascending order
  a = a.sort((a, b) => a - b);

  // Initialize a map to store the count of occurrences of each element
  const countMap = new Map();

  // Populate the countMap with the count of occurrences of each element
  a.forEach((num) => {
    countMap.has(num)
      ? countMap.set(num, countMap.get(num) + 1)
      : countMap.set(num, 1);
  });

  // Filter out elements with occurrences greater than 1
  let occurrenceNumber = a.filter((num) => countMap.get(num) > 1);

  // Convert the filtered array to a Set to remove duplicates
  occurrenceNumber = new Set(occurrenceNumber);

  // Check if the Set is not undefined, and return the elements in ascending order
  return occurrenceNumber !== undefined ? [...occurrenceNumber] : [-1];
}

Approach:

1. The array a is sorted in ascending order using the sort() method.

2. A Map named countMap is initialized to store the count of occurrences of each element in the array.

3. Using forEach(), each element of the array is iterated, and its count is updated in the countMap.

   1. countMap.has(num): This checks whether the num is already a key in the countMap.

      1. countMap.get(num) + 1: It fetches the current count of num and increments it by 1.

      2. countMap.set(num, countMap.get(num) + 1) it sets this new count as the value for the key num using countMap.set(num, ...).

4. The filtered array is converted to a Set to remove duplicate elements.

5. The set is converted back to an array using the spread operator [...occurrenceNumber], and returned. If no duplicate elements are found, [-1] is returned.