Two-Pointer Technique in a Linked List

The two-pointer technique is a common technique used in linked list problems to solve a variety of problems efficiently. It involves using two pointers that traverse the linked list at different speeds or in different directions.

Use Cases of Two-Pointer Technique in a Linked List:

1. Finding the Middle of a Linked List:

One common application of the two-pointer technique is finding the middle of a linked list. Here’s how to do it:

• Initialize both pointers to the head of the linked list.
• Move the fast pointer two nodes at a time, while moving the slow pointer one node at a time.
• When the fast pointer reaches the end of the linked list, the slow pointer will be at the middle node.

2. Finding a Loop in a Linked List:

Another application of the two-pointer technique is detecting loops in a linked list. Here’s how to do it:

• Initialize both pointers to the head of the linked list.
• Move the fast pointer two nodes at a time, while moving the slow pointer one node at a time.
• If the fast pointer ever catches up to the slow pointer, there is a loop in the linked list.

3. Reversing a Linked List:

The two-pointer technique can also be used to reverse a linked list. Here’s how to do it:

• Initialize three pointers:
  • Current pointer (curr): Points to the current node.
  • Previous pointer (prev): Points to the previous node.
  • Next pointer (next): Points to the next node.
• Set curr to the head of the linked list.
• While curr is not null:
  • Set next to the next node of curr.
  • Set curr’s next pointer to prev.
  • Set prev to curr.
  • Set curr to next.
• The head of the reversed linked list is now prev.

4. Finding the nth Node from the End of a Linked List:

To find the nth node from the end of a linked list using the two-pointer technique, follow these steps:

• Initialize two pointers, p and q, to the head of the linked list.
• Advance the pointer q, n nodes ahead of the pointer p.
• Now, advance both pointers p and q one node at a time.
• When the pointer q reaches the end of the linked list, the pointer p will be at the nth node from the end.

5. Finding the Intersection Point of Two Linked Lists:

To find the intersection point of two linked lists using the two-pointer technique, follow these steps:

• Initialize two pointers, p and q, to the heads of the two linked lists, respectively.
• Traverse the first linked list using the pointer p until you reach the end.
• Then, set the pointer p to the head of the second linked list.
• Similarly, traverse the second linked list using the pointer q until you reach the end.
• Then, set the pointer q to the head of the first linked list.
• Now, advance both pointers p and q one node at a time.
• If the two pointers ever meet, then the point of intersection is the node where they meet.
• If the two pointers reach the end of both linked lists without meeting, then there is no intersection point.

Conclusion:

The two-pointer technique is a powerful technique that can be used to solve a variety of problems involving linked lists efficiently. It involves using two pointers that traverse the linked list at different speeds or in different directions. By carefully coordinating the movement of the two pointers, you can skip over nodes, detect loops, reverse the linked list, and more.