Two-Pointer Technique: Solving Array Problems at Light Speed

Explained intuitively through examples.Two-pointer technique is commonly used to solve array problems very efficiently. Whenever an array question deals with finding two numbers in an array that satisfy a certain condition, either directly or indirectly, two-pointer should be the first strategy that comes to mind. This method rules out large numbers of redundant potential solutions.

Two-pointer technique is commonly used to solve array problems very efficiently. Whenever an array question deals with finding two numbers in an array that satisfy a certain condition, either directly or indirectly, two-pointer should be the first strategy that comes to mind. This method rules out large numbers of redundant potential solutions. Although it is tricky to construct, by following certain principles, two-pointer is a simple and powerful technique to implement.

Using two-pointer is best illustrated with an example. Consider the classic two-sum problem, which asks you to find which of two elements in an array adds to another number. Although there may be many solutions, it only asks for one. In this case, let the target by 13. One solution is (9, 3).

The brute force solution would be to run through every possible pair of elements, a solution that runs in O(n²) time. However, we can use two-pointer to narrow the runtime to O(n), a drastic speedup.

First, we sort the list, for reasons you will see soon:

Next, we place one pointer at the front and another at the end of the array. Because the list is sorted, this corresponds to the smallest and largest numbers, respectively.

For each step, we calculate the sum of the two numbers being pointed to.

If the sum is less than the target, we want to increase the estimate by moving the left pointer one to the right. This brings the estimate closer to the target value. If, however, we were to move the right pointer left, our estimate would have been brought down, leading to a redundant test case.

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