Revert "Revert "add solution to round trip example""

This reverts commit e7b4ada1.

exercises/MPI_examples/round_trip/round_trip.c

@@ -13,21 +13,25 @@ int main(int argc, char** argv) {
   // of the special case when you are the first process to prevent deadlock.
 
   if (rank != 0) {
-    // TODO: Receive the token from the previous process in the ring
+    MPI_Recv(&token, 1, MPI_INT, rank - 1, 0, MPI_COMM_WORLD,
+             MPI_STATUS_IGNORE);
     printf("Process %d received token %d from process %d\n", rank, token,
            rank - 1);
   } else {
-    // TODO: Set the token's value if you are process 0
+    // Set the token's value if you are process 0
+    token = 1024;
   }
 
-  // TODO: send the token to the next process in the ring 
+  MPI_Send(&token, 1, MPI_INT, (rank + 1) % num_procs, 0,
+           MPI_COMM_WORLD);
 
   // Now process 0 can receive from the last process. This makes sure that at
   // least one MPI_Send is initialized before all MPI_Recvs (again, to prevent
   // deadlock)
 
   if (rank == 0) {
-    // TODO: Receive the token from the last process in the ring
+    MPI_Recv(&token, 1, MPI_INT, rank - 1, 0, MPI_COMM_WORLD,
+             MPI_STATUS_IGNORE);
     printf("Process %d received token %d from process %d\n", rank, token,
            num_procs - 1);
   }