Thursday, April 8, 2010

Detecting a loop in a linked list (C program)

Brute force method

Have a double loop, where you check the node pointed to by the outer loop, with every node of the inner loop.

typedef struct node
  void *data;
  struct node *next;

mynode * find_loop(NODE * head)
  mynode *current = head;

  while(current->next != NULL)
    mynode *temp = head;
    while(temp->next != NULL && temp != current)
      if(current->next == temp)
        printf("\nFound a loop.");
        return current;
      temp = temp->next;
    current = current->next;
  return NULL;

Visited flag

Have a visited flag in each node of the linked list. Flag it as visited when you reach the node. When you reach a node and the flag is already flagged as visited, then you know there is a loop in the linked list.

Fastest method

Have 2 pointers to start of the linked list. Increment one pointer by 1 node and the other by 2 nodes. If there's a loop, the 2nd pointer will meet the 1st pointer somewhere. If it does, then you know there's one.

Here is some code


while(p!=NULL && q!=NULL)
    //Loop detected!

// No loop.

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