Recognizing & Mastering Coding?Patterns - Part 1: Two Pointer Technique (Opposite Ends)

One thing becomes clear as we examine coding interviews in greater detail: it’s critical to recognize patterns. Understanding these crucial problem-solving patterns will help us identify common structures and use tried-and-true solutions instead of constantly starting from scratch. Among these is the Two Pointer Technique, which is both effective and powerful.

In this article, I’m kicking off the first installment of a series that explores different approaches to using the two-pointer method, starting with pointers at opposite ends.I’ll go over the fundamentals of this pattern, including what the method is, when and where to use it, and a carefully selected collection of LeetCode problems that are easy for beginners to understand. Stay tuned for more articles where I’ll dive into other essential strategies!

What is the Two-Pointer Technique?

The two-pointer technique is a problem-solving method that involves traversing an array or list with two pointers. Depending on the nature of the problem, these pointers may begin with:

1. Opposite ends (which I will discuss in this post ).
2. The same starting point (covered in the next installment of this series).

This method is extremely effective when dealing with problems involving sorted arrays, subarrays, palindromes, or pair sums.

Two Pointers from Opposite?Ends

In problems where the array or string is sorted or involves comparison from both ends (e.g., checking for palindromes or reversing specific elements), it’s often useful to start the two pointers at opposite ends and move them towards each other. This allows you to narrow down the search space, making the solution much more efficient.

When to Use Two Pointers from Opposite?Ends?

There are some obvious clues in a problem description that suggest it might be best to use two pointers from opposite ends:

1. Sorted Array or String:

• If the input is sorted, it’s a good clue that the two-pointer technique could work, as you can exploit the order of elements to compare and narrow down possibilities.
• Example: Finding pairs in a sorted array that sum to a target value.

2. Pair, Triplet, or Subarray Problems:

• If the problem asks you to find pairs, triplets, or certain subarrays that meet a specific condition (like a target sum or difference), using two pointers often optimizes the search.
• Example: Finding triplets in a sorted array that sum to zero.

3. Comparing Opposite Ends (Palindrome, Reverse):

• If you’re required to check if a string or array is a palindrome, or you need to reverse specific parts of it, you’ll often compare elements from both ends.
• Example: Checking if a string is a valid palindrome by ignoring non-alphanumeric characters.

4. Maximum or Minimum Difference Between Two Elements:

• When the task involves maximizing or minimizing the difference between two elements in a sorted array, two pointers can help efficiently adjust and compare values.
• Example: Finding the maximum container that can hold water between two heights.

Tips for Solving Problems with Two Pointers at Opposite?Ends

1. Sorted Inputs: Always check whether the array or string is sorted. If it is, it’s a good sign to try the two-pointer technique
2. Increment and Decrement Carefully: Typically, one pointer moves forward and the other backward. If the sum is too small, increase the left pointer. If the sum is excessively large, decrement the right pointer.
3. Avoid Overlapping: Make sure your two pointers do not cross each other unless it is the stopping condition. This helps to avoid missing valid pairs and miscalculating conditions.
4. Edge Cases: Watch out for arrays that contain all the same elements or just one element.

Example LeetCode?Problems

These EASY LeetCode problems, which heavily rely on the two-pointer technique with pointers starting at opposite ends, will help you get a grip of this method:

• Two Sum II—Input Array is Sorted—Find two numbers in a sorted array that add up to a target value.
• Valid Palindrome?—?Check if a string is a valid palindrome, considering only alphanumeric characters.
• Reverse Vowels of a String?—? Reverse only the vowels in a string.
• Squares of a Sorted Array?—?Return an array of the squares of each number in sorted order.
• Valid Palindrome II?—?Determine if a string can be a palindrome by removing at most one character.
• Merge Sorted Array?—?Merge two sorted arrays into one in place.
• Move Zeroes?—?Move all zeroes in the array to the end while maintaining the order of non-zero elements.

Why Pattern Recognition is Important

Understanding patterns like the two-pointer technique is essential for coding interviews and perfecting your problem-solving abilities. Instead of viewing each problem as an isolated incident, you’ll begin to recognize the underlying structure, allowing you to approach problems with confidence and efficiency.

In the next part of this series, I’ll go over how to use two pointers that start at the same end, which is a slightly different approach that solves a wide range of problems.

Conclusion

One of the most effective and adaptable coding techniques you have is the two-pointer technique. Today, we talked about how to use the technique with pointers at opposite ends and looked at some sample problems to get you started. I encourage you to test these on LeetCode to solidify your grasp of the pattern. In the next article, we’ll look at two pointers that start at the same end, opening up a whole new set of possibilities. Stay tuned for additional coding interview tips and patterns!

Request for?Action

You can’t master it without trying it! Put these patterns into practice right now. And I mean NOW!!

In the following section, we’ll look at even more strategies for solving problems involving two pointers. I’ll keep it concise to make it easy for you to revisit during revision. I’m also at a beginner-to-medium level, so feel free to reach out with any suggestions or tips. Don’t forget to share your experiences and let me know in the comments how these techniques have helped in your coding journey!

Thank you for sticking with me until the end! Keep pushing forward, and remember that practice makes perfect! ??