Sliding Window Technique 10 XP Easy

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The Sliding Window technique is one of the most powerful array/string patterns! 🪟

💡 Think of it like this: Imagine looking at a city through a window on a moving train. The window shows a portion of the city, and as the train moves, the view shifts. In coding, our "window" is a range of elements in an array, and we slide it along!

💭 Go for it! Apply the approach above in your own code. If you get stuck, use the hint below! 🔥

Two types of sliding windows:

📏 Fixed-size: Window always has exactly k elements. Slide by adding right, removing left.
↔️ Variable-size: Window grows/shrinks based on a condition. Use two pointers (left, right).
🔑 Key insight: We're reusing work from the previous window instead of starting over!
⚡ Turns O(n × k) brute force into O(n) — a huge speedup for large inputs.

Variable-size window template:

def variable_window(arr):
    left = 0
    for right in range(len(arr)):
        # Expand window: add arr[right]
        while window_is_invalid():
            # Shrink window: remove arr[left]
            left += 1
        # Update answer with current valid window

📋 Instructions

Implement `max_sum_subarray(nums, k)` that finds the maximum sum of any `k` consecutive elements. Test with `nums = [2, 1, 5, 1, 3, 2]` and `k = 3`. Print the result.

Calculate sum of first k elements. Then slide: add nums[i], subtract nums[i-k]. Track the maximum sum.

⚠️ Try solving it yourself first — you'll learn more!

# Fixed-size window: Find max sum of k consecutive elements
def max_sum_k(arr, k):
    window_sum = sum(arr[:k])    # First window
    max_sum = window_sum
    
    for i in range(k, len(arr)):
        window_sum += arr[i]      # Add new element (right side)
        window_sum -= arr[i - k]  # Remove old element (left side)
        max_sum = max(max_sum, window_sum)
    
    return max_sum
# Instead of recalculating sum for every window (O(n×k)),
# we just add one and remove one (O(n))!

🧪 Test Cases

Input

max_sum_subarray([2, 1, 5, 1, 3, 2], 3)

Expected

9

Test case 1

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main.py

Hi! I'm Rex 👋

Output

Ready. Press ▶ Run or Ctrl+Enter.

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