Sliding Window as a Reusable Pattern

Recognition

Sliding windows apply when the state of a contiguous range can be updated as one item enters and another leaves.

• Fixed-size windows
• Longest valid range
• Minimum satisfying range

The invariant

Define exactly what the current window stores and when it is valid. The loop then expands or contracts while preserving that rule.

• Counts
• Running sum
• Constraint violations

Complexity

When both pointers only move forward, each element enters and leaves at most once, producing linear time.