Two Pointer and Sliding Window Algorithm

Introduction

• This video introduces the Two Pointer and Sliding Window playlist from the Strivers A to Z Data Structures.
• The goal is to understand the concepts rather than memorizing algorithms.
• Coverage of four different problem patterns and templates for solving problems.
• Importance of watching the video till the end to grasp the patterns and templates.

Problem Patterns

Pattern 1: Constant Window

• Example Problem: Given an array with positive and negative integers, and an integer k. Find the maximum sum of k consecutive elements.
• Solution Approach:
  • Use two pointers, L and R, to denote the current window.
  • Compute initial sum from index 0 to k-1.
  • Shift the window by moving L and R, adjusting the sum accordingly:
    • Move L to L+1, remove the element at L from the sum.
    • Move R to R+1, include the element at R in the sum.
  • Compare the sum to find the maximum.

Pattern 2: Longest Subarray or Substring

• Example Problem: Find the longest subarray where the sum is less than or equal to k.
• Brute Force:
  • Generate all possible subarrays and check their sums.
• Sliding Window Approach:
  • Start with a window size of one using pointers L and R.
  • Expand R to include elements until the sum exceeds k.
  • If the sum exceeds k, shrink from the left (L) until the sum is valid again.
  • Keep track of max length of valid subarrays.
• Time Complexity: O(n) if implemented correctly with the above methods.

Pattern 3: Number of Subarrays with a Condition

• Example Problem: Count the number of subarrays with a given sum.
• Approach involves breaking it down into two separate counts:
  • Count subarrays with sum <= k.
  • Count subarrays with sum <= k-1.
  • The answer is the difference of the two counts.

Pattern 4: Shortest or Minimum Window

• Very rare problems that require finding the minimum size window under certain conditions.
• Approach: Start with a valid window and try to shrink it to find the minimum size while maintaining validity.

Conclusion

• The video serves as an introductory overview to the Two Pointer and Sliding Window techniques.
• Emphasizes understanding the problem patterns and applying the templates to solve various problems effectively.
• Encouragement to practice with 10-12 problems to gain confidence for interviews or coding rounds.
• Reminder to like and subscribe to the channel for more content.