Find Duplicate Elements in an Array Using Hash Map

function findDuplicates(arr) {
  const map = {};
  const res = [];
  for (let num of arr) map[num] = (map[num] || 0) + 1;
  for (let num in map) if (map[num] > 1) res.push(Number(num));
  return res;
}
                  

Explanation: Count frequencies using a hash map and collect elements with count > 1.

⏱ O(n) | 💾 O(n)

class Solution {
  public List<Integer> findDuplicates(int[] arr) {
    Map<Integer,Integer> map = new HashMap<>();
    List<Integer> res = new ArrayList<>();
    for (int num:arr) map.put(num,map.getOrDefault(num,0)+1);
    for (int key:map.keySet()) if(map.get(key)>1) res.add(key);
    return res;
  }
}
                  

Explanation: HashMap counts duplicates and keys with count > 1 are returned.

⏱ O(n) | 💾 O(n)

def find_duplicates(arr):
    count = {}
    res = []
    for num in arr: count[num] = count.get(num,0)+1
    for num in count:
        if count[num]>1: res.append(num)
    return res
                  

Explanation: Dictionary counts frequency; elements with count > 1 are duplicates.

⏱ O(n) | 💾 O(n)