Sliding window works on substrings/subarrays where you need a sum, a count, or a property (no repeats, all chars present). You maintain L and R pointers and a window-state dict; advance R to grow, advance L to shrink on violation. O(N) overall because each element enters and leaves once.
Max sum subarray length K: window sum updates as you slide.
Longest substring without repeating: window dict of last seen index, L jumps.
Minimum window substring: shrink the window as long as all required chars are present.
arr -> grow R until condition met -> shrink L until condition broken -> record answer -> repeatSliding window when you fix a window size or a condition. Two-pointer when ordered pairs.