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// Slow: nested loop to find duplicates
function findDuplicates(arr) {
  var duplicates = [];
  for (var i = 0; i < arr.length; i++) {
    for (var j = i + 1; j < arr.length; j++) {
      if (arr[i] == arr[j]) {
        duplicates.push(arr[i]);
      }
    }
  }
  return duplicates;
}

// Fast: Set-based O(n) approach
function findDuplicates(arr) {
  const seen = new Set();
  const duplicates = new Set();

  for (const item of arr) {
    if (seen.has(item)) {
      duplicates.add(item);
    }
    seen.add(item);
  }

  return [...duplicates];
}