Coding Interview Patterns

Master the essential patterns that appear in 90% of coding interviews with visual explanations and step-by-step examples.

What You'll Learn

• Two Pointers: Efficiently solve array and string problems

• Sliding Window: Handle subarray and substring problems

• Fast & Slow Pointers: Detect cycles and find middle elements

• Merge Intervals: Combine overlapping ranges

• Cyclic Sort: Sort arrays with specific constraints

• In-place Reversal: Reverse linked lists and arrays efficiently

Preview Chapter: Two Pointers

The two pointers technique is one of the most powerful patterns in coding interviews. It allows you to solve many array and string problems in O(n) time complexity.

When to Use Two Pointers

• Sorted arrays: When you need to find pairs or triplets

• Palindrome problems: Check if a string is a palindrome

• Container problems: Find maximum area or volume

• Remove duplicates: Clean up arrays efficiently

Example: Two Sum

def two_sum_sorted(nums, target):
    left, right = 0, len(nums) - 1
    
    while left < right:
        current_sum = nums[left] + nums[right]
        
        if current_sum == target:
            return [left, right]
        elif current_sum < target:
            left += 1
        else:
            right -= 1
    
    return []

Visual Explanation

Array: [2, 7, 11, 15], Target: 9

Step 1: left=0, right=3
        [2, 7, 11, 15]
         ↑         ↑
         left    right
        sum = 2 + 15 = 17 > 9, move right

Step 2: left=0, right=2
        [2, 7, 11, 15]
         ↑     ↑
         left right
        sum = 2 + 11 = 13 > 9, move right

Step 3: left=0, right=1
        [2, 7, 11, 15]
         ↑  ↑
         left right
        sum = 2 + 7 = 9 = target ✓

Course Structure

1. Two Pointers Pattern (2 hours)

- Basic two pointers

- Container with most water

- Three sum problem

- Practice exercises

2. Sliding Window Pattern (1.5 hours)

- Fixed window size

- Variable window size

- Longest substring problems

- Practice exercises

3. Fast & Slow Pointers (1.5 hours)

- Cycle detection

- Find middle element

- Palindrome detection

- Practice exercises

4. Merge Intervals (1 hour)

- Basic merging

- Insert intervals

- Meeting rooms

- Practice exercises

5. Cyclic Sort (1 hour)

- Basic cyclic sort

- Find missing numbers

- Find duplicates

- Practice exercises

6. In-place Reversal (1 hour)

- Reverse linked list

- Reverse array

- Reverse subarray

- Practice exercises

Practice Problems

Each pattern includes 5-10 carefully selected practice problems with:

• Detailed solutions

• Time and space complexity analysis

• Common pitfalls and how to avoid them

• Interview tips and tricks

What's Next

After mastering these patterns, you'll be ready to tackle any coding interview with confidence. These patterns form the foundation for solving 90% of coding interview problems.