69-two-pointers-pattern

The two pointers pattern is a common technique used in many LeetCode problems to optimize the time and space complexity of algorithms, especially when dealing with arrays or strings. The idea is to use two distinct pointers that traverse the data structure, often from opposite ends or both starting from the same point, depending on the problem’s requirements. By carefully controlling the movement of these pointers, we can avoid unnecessary operations, reduce complexity, and efficiently solve problems that would otherwise require more brute-force approaches.

One typical use case of the two pointers technique is in problems that involve finding pairs or subarrays that satisfy certain conditions, like summing up to a target value, checking for symmetry, or sorting elements. For instance, in the classic “Two Sum II - Input Array Is Sorted” problem, one pointer starts at the beginning of the array and the other at the end. The two pointers move inward based on the sum of the values they point to, either increasing or decreasing the sum until the target is found. This results in an O(n) time complexity, which is much more efficient than the brute-force O(n²) approach of checking every pair.

Another example is the “Valid Palindrome” problem, where we check if a given string is a palindrome. Here, two pointers can be placed at the beginning and end of the string, comparing characters as they move towards the center. If any characters don’t match, the string is not a palindrome. This method reduces the need to reverse the string or use additional space, making the solution more optimal.

In summary, the two pointers pattern is a powerful technique that reduces the complexity of many problems, especially those related to arrays and strings, by leveraging simultaneous traversal with two pointers. It’s widely applicable in problems involving sorting, searching, and comparisons.