Sliding window solves: find the optimal contiguous subarray/substring satisfying some constraint.

Brute force generates every subarray: O(n²) or O(n³). Sliding window maintains a dynamic range [left, right] that expands right, then shrinks left — always processing each element at most twice: O(n).

Two variants:

1. Fixed size window — window size k is given. Slide the window right, add the new element, drop the leftmost.

2. Variable size window — expand right until the constraint is violated, then shrink left until valid again.

The signal:

• Contiguous subarray or substring
• Maximise / minimise something (length, sum, distinct characters)
• O(n) solution should exist

One word matters above all: contiguous. If the problem says "subsequence" (elements may be skipped), a window won't work — windows only ever cover an unbroken range.