Sliding window

Sliding Window

syntax

Maintain a window [left..right] and expand/shrink to satisfy a condition.
Fixed-size window: move both pointers together.
Variable-size window: expand right, shrink left when constraint violated.

example

// JavaScript — Max sum subarray of size k
function maxSumWindow(arr, k) {
  let windowSum = 0, maxSum = -Infinity;
  for (let i = 0; i < arr.length; i++) {
    windowSum += arr[i];
    if (i >= k) windowSum -= arr[i - k];
    if (i >= k - 1) maxSum = Math.max(maxSum, windowSum);
  }
  return maxSum;
}

# Python — Max sum subarray of size k
def max_sum_window(arr, k):
    window_sum = 0
    max_sum = float('-inf')
    for i in range(len(arr)):
        window_sum += arr[i]
        if i >= k:
            window_sum -= arr[i - k]
        if i >= k - 1:
            max_sum = max(max_sum, window_sum)
    return max_sum

output

max_sum_window([2,1,5,1,3,2], 3) → 9 (subarray [5,1,3])

Note Time O(n), Space O(1). Sliding window converts O(n*k) brute force to O(n). For variable-size windows (e.g., longest substring without repeats), use a hash set to track window contents. Always clarify: is the window fixed or variable size?

SQLSQL

Window Frame Specification

SQL · Window Functions

syntax

ROWS BETWEEN start AND end
-- start/end: UNBOUNDED PRECEDING | n PRECEDING | CURRENT ROW | n FOLLOWING | UNBOUNDED FOLLOWING

example

SELECT
  sale_date,
  amount,
  AVG(amount) OVER (
    ORDER BY sale_date
    ROWS BETWEEN 6 PRECEDING AND CURRENT ROW
  ) AS weekly_moving_avg,
  SUM(amount) OVER (
    ORDER BY sale_date
    ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
  ) AS cumulative_total
FROM daily_sales;

output

-- 7-day moving average alongside cumulative total

Note ROWS counts physical rows. RANGE groups rows with the same ORDER BY value together. For date-based ranges in PostgreSQL, you can use RANGE BETWEEN INTERVAL '7 days' PRECEDING AND CURRENT ROW if the ORDER BY column is a date.

DSAInterview Prep

SQLSQL