LC-TEST-09 — Sliding Window Like a Pro

🎯 Why Sliding Window Is a Game Changer

Sliding Window is what happens when:

Two pointers learn to cooperate.

This technique is everywhere:

• Subarrays
• Substrings
• Streaming data
• Real-time analytics

Interviewers love this topic.

🧠 What You Will Learn

This lesson trains you to:

• Expand and shrink windows deliberately
• Track state using containers (dict, set)
• Replace nested loops with one clean pass
• Write code that looks effortlessly smart

🧩 Core Example — Minimum Window Substring

Problem Statement

Given strings s and t, return the minimum window in s that contains all characters of t.

If no such window exists, return "".

📥 Sample Input / 📤 Output

🧠 Interviewer Insight

❗ This problem tests:

• Can you track frequencies
• Do you know when a window becomes valid
• Can you shrink without breaking correctness

If you brute force substrings → instant fail.

🧩 Sliding Window Strategy

💡 Mental Model

• Right pointer expands the window
• Left pointer shrinks it
• A container tracks validity

Never reset. Never restart loops.

🧑‍💻 Sliding Window Solution

from collections import Counter

def minWindow(s, t):
    need = Counter(t)
    missing = len(t)
    left = start = end = 0

    for right, ch in enumerate(s, 1):
        if need[ch] > 0:
            missing -= 1
        need[ch] -= 1

        if missing == 0:
            while left < right and need[s[left]] < 0:
                need[s[left]] += 1
                left += 1

            if end == 0 or right - left < end - start:
                start, end = left, right

            need[s[left]] += 1
            missing += 1
            left += 1

    return s[start:end]

⏱️ Complexity

• Time: O(n)
• Space: O(k) (character set)

This is optimal and interview-approved.

🧠 Sliding Window Patterns

🧩 Beginner vs Pro Thinking

Beginner:

“Let me try all substrings.”

Pro:

“I expand until valid, then shrink carefully.”

That mindset shift is what interviews test.

🎯 Final Takeaway

Sliding Window is not hard.

Restarting loops is.

If your window never resets, your solution is probably correct.

🏁 Practice Targets

• Longest Substring Without Repeating Characters
• Permutation in String
• Max Consecutive Ones III
• Subarray Sum Equals K
• Find All Anagrams in a String

Master these → you officially think like a LeetCode veteran 🚀