Added code for Friends Pairing problem

Author: sandeep-07

20. Dynamic Programming/Friend_Pairing._Problem.cpp

@@ -0,0 +1,52 @@
+//Implementing Friend-Pairing problem using dynamic programming
+//hacktoberfest2021
+
+// https://practice.geeksforgeeks.org/problems/friends-pairing-problem5425/1#
+#include <bits/stdc++.h> 
+using namespace std; 
+
+ // } Driver Code Ends
+class Solution
+{
+public:
+    //Function to return the total number of ways in which 'n' friends can remain single or can be paired up.
+    int countFriendsPairings(int n) 
+    { 
+        // Your code here
+        long long int mod=1000000000+7;
+        long long int dp[n+1];
+        for(int i=0;i<=n;i++)
+        {
+            if(i<=2)
+            {
+                dp[i]=i%mod;
+            }
+            else
+            {
+                dp[i]=(dp[i-1]%mod+(i-1)*(dp[i-2]%mod))%mod;
+            }
+        }
+        
+        return dp[n]%(mod);
+    }
+};    
+ 
+
+// { Driver Code Starts.
+int main() 
+{
+    //taking total testcases
+    int t;
+    cin>>t;
+    while(t--)
+    {
+        // reading total friends
+    	int n;
+    	cin>>n;
+    	Solution ob;
+        //calling method countFriendsPairings
+    	cout <<ob.countFriendsPairings(n); 
+    	cout<<endl;
+    }
+} 
+  // } Driver Code Ends