LC-TEST-08 — Two Pointers in the Wild

🎯 Why Two Pointers Matter

Two pointers is not a trick.
It is a way of thinking.

Strong developers see:

“I don’t need extra memory.
I can let indices move and converge.”

This technique appears everywhere:

• Arrays
• Strings
• Sorted data
• Geometry problems
• Optimization tasks

If you master this, your code gets shorter immediately.

🧠 What You Will Learn

In this lesson, you will train to:

• Use left / right pointers confidently
• Decide who moves and why
• Avoid nested loops
• Reduce O(n²) → O(n)

This is interview-critical muscle memory.

🧩 Core Example — Container With Most Water

Problem Statement

Given n non-negative integers representing vertical lines, find two lines that together with the x-axis form a container that holds the most water.

📥 Sample Input / 📤 Output

🧠 Interviewer Insight

❗ Interviewers want to see:

• Do you understand why one pointer moves
• Can you explain why brute force is wasteful
• Do you trust a greedy decision?

If you randomly move pointers, you fail. If you justify movement, you pass.

🧩 Brute Force (What NOT To Do)

max_area = 0
for i in range(len(height)):
    for j in range(i + 1, len(height)):
        max_area = max(
            max_area,
            min(height[i], height[j]) * (j - i)
        )

⛔ O(n²) ⛔ Interview red flag

🧩 Two Pointer Strategy (Correct)

💡 Core Insight

The shorter line limits the area.

So:

• Move the pointer pointing to the shorter height
• Because moving the taller one cannot increase area

This is the entire trick.

🧑‍💻 Two Pointer Solution

def maxArea(height):
    left, right = 0, len(height) - 1
    best = 0

    while left < right:
        width = right - left
        area = min(height[left], height[right]) * width
        best = max(best, area)

        if height[left] < height[right]:
            left += 1
        else:
            right -= 1

    return best

⏱️ Complexity

• Time: O(n)
• Space: O(1)

Optimal.

🧠 Two Pointer Patterns You Must Recognize

🎯 Final Takeaway

Two pointers is about controlled movement, not guessing.

If you can explain why a pointer moves, interviewers assume you understand algorithmic reasoning.

🏁 Practice Targets

• Valid Palindrome
• Two Sum II (sorted)
• Remove Duplicates
• 3Sum
• Squares of a Sorted Array

Master these → you look dangerous in interviews 😈