Refactor findCommonItems using a Set for faster lookups

Author: Fatma Degirmenci

Sprint-1/JavaScript/findCommonItems/findCommonItems.js

@@ -1,14 +1,16 @@
 /**
  * Finds common items between two arrays.
  *
- * Time Complexity:
- * Space Complexity:
- * Optimal Time Complexity:
- *
+ * Time Complexity: We are checking two arrays. If the first array has length n and the second has length m, the time complexity is O(n * m), 
+   because filter iterates over the first array and includes scans the second array for each element.
+ * Space Complexity: A new Set is created to store unique common items, which in the worst case can be the size of the first array --> O(n).
+ * Optimal Time Complexity: By converting one array to a Set, we can check membership in O(1) for each element of the other array, reducing the time complexity to O(n + m).
  * @param {Array} firstArray - First array to compare
  * @param {Array} secondArray - Second array to compare
  * @returns {Array} Array containing unique common items
  */
-export const findCommonItems = (firstArray, secondArray) => [
-  ...new Set(firstArray.filter((item) => secondArray.includes(item))),
-];
+export const findCommonItems = (firstArray, secondArray) => {
+  const firstArr = new Set(firstArray); //O(n)
+  const commons = secondArray.filter((el) => firstArr.has(el)); //O(m)
+  return [...new Set(commons)]; // remove duplicates
+};