Added new algorithm/problem in Array-DS

Data-Structures/Array/SingleElement.js

@@ -0,0 +1,25 @@
+/** https://leetcode.com/problems/single-number/description/
+ * This function will accept an array and
+ * Find an element which has only one frequency among other duplicate elements
+ * The solution is based on using two loops, if the array is large, sorting or hashing is necessary, we can use XOR operator or sum method too
+ * @param {Array} arr array with elements of integer type
+ * @returns {Number} with single element
+ */
+
+function SingleElement(arr) {
+  let n = arr.length
+  for (let i = 0; i < n; i++) {
+    let count = 0
+    // previous elements are already checked
+    for (let j = 0; j < n; j++) {
+      if (arr[j] === arr[i]) {
+        count++
+      }
+    }
+    if (count === 1) {
+      return arr[i]
+    }
+  }
+}
+
+export { SingleElement }

Data-Structures/Array/test/SingleElement.test.js

@@ -0,0 +1,11 @@
+import { SingleElement } from '../SingleElement'
+
+describe('single element in an array of duplicates', () => {
+  test.each([
+    [[1, 2, 2, 3, 3], 1],
+    [[8, 9, 7, 2, 1, 5, 6, 4, 5, 9, 8, 7, 2, 6, 1], 4],
+    [[10, 8, 5, 6, 8, 5, 6, 10, 11], 11]
+  ])('returns %p when given %p', (array, expected) => {
+    expect(SingleElement(array)).toBe(expected)
+  })
+})