Identifying the two pointer pattern

The two-pointer technique can solve some specific types of binary search tree problems. These are generally easy or medium problems in which we need to traverse the nodes in a binary search tree in the sorted and reverse-sorted order simultaneously until some terminating condition is reached. We may also have a function f that determines whether we should move ahead in the forward and/or reverse direction. If the problem statement or its solution follows the generic template below, it can be solved by applying the two-pointer technique.

Template:
Given a binary search tree, traverse the nodes simultaneously in the sorted and reverse sorted order until some terminating condition is reached. Process the nodes in each iteration, and based on the output of some function f, move ahead in the forward and/or reverse direction.

Example

Let's consider the following problem as an example to better understand how to identify and solve a problem using the two-pointer technique.

Problem statement: Given a binary search tree and a `target`, find if there is a pair of nodes with a sum equal to `target`.