Added hashing.cpp code

Author: ashish5433

hashing.cpp

@@ -0,0 +1,21 @@
+// Hashing help us to count the frequency of an element in the array in time complexity of O(N). so this is 
+// an efficient method for calculating the frequency of an element in array
+
+
+#include<bits/stdc++.h>
+using namespace std;
+const long long int M=1e5+5;
+int main(){
+int n; //size of the array
+cin>>n;
+int a[n];
+int hash_arr[M];
+for(int i=0;i<M;i++)hash_arr[i]=0;
+for(int i=0;i<n;i++){
+    cin>>a[i];
+    hash_arr[a[i]]++;
+}
+int x; // Number whose frequency we want
+cin>>x;
+cout<<hash_arr[x];
+}