How does the runner technique work in linked lists?

The runner technique in linked lists involves using two pointers moving at different speeds to traverse the list.

The runner technique, also known as the two-pointer technique, is a popular method used in linked list problems. It involves two pointers, usually referred to as the 'slow' pointer and the 'fast' pointer (or 'runner'). The slow pointer moves one step at a time, while the fast pointer moves two steps at a time. This technique is particularly useful in identifying certain properties of a linked list, such as detecting a cycle or finding the middle element.

The basic idea behind the runner technique is that by the time the fast pointer reaches the end of the list, the slow pointer will be halfway through (in case of no cycle). If there is a cycle in the list, the fast pointer will eventually catch up with the slow pointer, thus indicating the presence of a cycle. This technique can also be used to find the middle element of a linked list. When the fast pointer reaches the end of the list, the slow pointer will be at the middle.

To implement the runner technique, you initialise two pointers at the head of the list. Then, in each iteration of a loop, you move the slow pointer one step and the fast pointer two steps. If at any point the fast pointer becomes null (or in some cases, the next node of the fast pointer is null), it means you've reached the end of the list. If the fast pointer becomes equal to the slow pointer at any point, it indicates a cycle in the list.

The runner technique is a powerful tool for solving linked list problems, and it's a good idea to become familiar with it. It's a clever way of traversing a linked list using two pointers at different speeds, and it can help you solve problems more efficiently.