Understanding the Two Pointer Technique

The two-pointer approach is a common technique for solving sorted array and linked list problems. It involves using two pointers that traverse the data structure in a synchronized manner to solve the problem efficiently.

Process

1. Initialization: Start by initializing two pointers. The pointers can start at the beginning and end of the array or both at the beginning, but with one moving faster.

2. Traversal: Move the pointers based on the problem's requirements. For example, one pointer might move one step at a time while the other moves two, or both might move towards each other.

3. Condition Check: At each step, check if the pointers meet the condition required to solve the problem. This could be finding a pair of elements that sum to a target value or a subarray that meets certain criteria.

4. Termination: The process terminates when the pointers meet the termination condition, which could be when they cross each other when they find a solution or when they have both traversed the entire data structure.

When to Use

1. Finding Pairs: To find pairs of elements in an array that satisfy a certain condition (e.g., sum to a target value).

2. Finding Subarrays: To find subarrays that meet certain conditions, like having a specific sum.

How Does It Reduce Time Complexity?

1. Linear Time Complexity: The two-pointer approach often reduces the time complexity from O(n^2) (which would be typical with nested loops) to O(n). This is because each pointer traverses the array only once, or they converge towards each other.

2. Efficient Pair Checking: Instead of checking all possible pairs (which would require a nested loop), the two pointers efficiently find pairs by moving towards each other, reducing the number of comparisons.

Example problem for better understanding

Two Sum II - Input Array Is Sorted

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