Breadth First Search

void DReachable(int *A, int *B, int *W, int *LOC, int *DEG, int *A2, int *B2, int *W2, int *LOC2, int *DEG2)
{
 int K,L,N,Nnodes,CNT,
       *P,*Q,*PP,*QQ,      BUF1[MM],BUF2[MM],Nstate[MM];

 Nnodes=B[0];

/********* Part 1: nodes reachable from entry node 1 ***********/

 for (N=0; N<=Nnodes; N++)  Nstate[N]=0;
 BUF1[0]=1; BUF1[1]=0; PP=BUF2; QQ=BUF1;

MAINLP:
 P=QQ; Q=PP; PP=P; QQ=Q;     /* switch pointers to buffers */

 K=0;
 if (!*P)
  {       /* no more unlabeled nodes available */
   puts("\nThe following nodes are reachable from node 1:");
   for (N=1; N<=Nnodes; N++) if (Nstate[N]) {printf("%d ",N); K++;}   puts("");
   if (K<Nnodes)
    {
     puts("The following nodes are NOT reachable from node 1:\n");
     for (N=1; N<=Nnodes; N++) if (!Nstate[N]) {printf("%d ",N);} puts("");
     puts("DiGraph is not a transport network.");
    }
   else puts("From entry node 1 all nodes are reachable.\n");
  goto PART2;
  }

 while (*P)
  {               /* run through all nodes in buffer to which P points */
   Nstate[*P]=2;
   L=LOC[*P]; CNT=DEG[*P];
   while (CNT)
    {
     if (!Nstate[B[L]]) {*Q=B[L]; Nstate[*Q]=1; Q++;}
     L++; CNT--;
    }
   P++;
  }
 *Q=0;     /* mark end of nodes in other buffer containing neighbors */
 goto MAINLP;