Why not just sum both arrays up the difference will be the different no.

Using HashTable to count the occurrence of each number

In Python: from collections import Counter def diffNum(a,b): aCount = Counter(a) for x in b: if x not in aCount: return x return None a = [1,3,4,5,2] b = [1,2,7,5,4,3] print diffNum(a,b)

Given 2 lists a and b with one element that is different, sum(a) - sum(b) will give you the element

Here is the C# code with HashTable class Class1 { public int Different_Number(int[] A, int[] B) { if (A.Length < B.Length) { return Hash_It(A, B); } else { return Hash_It(B, A); } } private int Hash_It(int[] A, int[] B) { Hashtable ht = new Hashtable(); for (int i = 0; i < A.Length; i++) { ht.Add(A[i], i); } for (int i = 0; i < B.Length; i++) { if (!ht.Contains(B[i])) { return B[i]; } } return null; } public static void Main() { int[] A = new int[] { 1,5,3,8}; /* Unsorted array */ int[] B = new int[] { 5,3,8,2102,1}; Class1 cls = new Class1(); Console.WriteLine(cls.Different_Number(A, B)); Console.ReadLine(); } }