Checks whether a single linked list contains loops.

Given a single-chain table, check whether the linked list contains a ring structure. This topic is mainly based on the characteristics of the ring, using two pointers with different step sizes to move forward in one direction. If there is a ring, the two pointers must meet each other.

 # Include  <  Stdio. h  >  
# Include  <  Malloc. h  >  

Typedef  Struct  Lnode {
  Int  Data;
  Struct  Lnode *  Next;
} Lnode;

  Int  Isloop (lnode  *  Head ){
Lnode  *  P  =  Head,  *  Q  =  Head;
  While  (Q  && Q  ->  Next ){
P  =  P  ->  Next;
Q  =  Q  ->  Next  ->  Next;
  If  (P  =  Q) Break  ;
}

  Return     !  (Q  =  Null  |  Q  ->  Next  =  Null );
}

  Int  Main (){
Lnode  * Head  =  (Lnode  *  ) Malloc (  Sizeof  (Lnode ));
Lnode  *  Curr  =  Head;
Lnode  *  Start  =  NULL;
Head  ->  Next =  NULL;
  Int  I;
  For  (I  =  0  ; I  <  10  ; I  ++  ){
Curr  ->  Next  =  (Lnode *  ) Malloc (  Sizeof  (Lnode ));
Curr  =  Curr  ->  Next;
Curr  ->  Next  =  NULL;
  If  (I  =  5  ) Start =  Curr;
}
Curr  ->  Next  =  Start;
Printf (  "  % D  "  , Isloop (head ));
}