Sliding Window

Maintain an aggregate over every length-k contiguous subarray by adding the entering element and removing the leaving one, turning O(n·k) into O(n).

Grow a window from the right while a condition holds and shrink it from the left when it breaks, finding the longest or shortest valid contiguous subarray in O(n).

Slide a window over a string while a hash map tracks character counts and a "formed" counter says how many required characters are satisfied — the engine behind anagram search and minimum window substring.