Stack

LeetCode 32 note: a stack of indices whose bottom is the position just before the current valid run; the length is i - stack[-1]

LeetCode 331 note: count open slots: start with 1, every node fills one, a non-null node opens two more; never go negative and end at exactly 0

LeetCode 173 note: flattening to an array is simple but uses O(n) space; the follow-up keeps the left spine on a stack for amortized O(1) and O(h) space

Parsing nested structures, simulations that interact with the previous element, monotonic stacks, greedy deletion for the smallest string, and a lazy-deletion Max Stack

LeetCode 2751 note: sort by position, push right-moving robots, and let each left-moving robot collide until one side is gone

LeetCode 4045 note: merge robots already close at t = 0, then go right to left like car fleets, where anything faster than the fleet ahead eventually joins it

LeetCode 716 note: store unique ids on both the stack and the heap, record deletions by id, and discard deleted entries lazily; tracking values cannot tell duplicates apart

LeetCode 1028 note: a stack holds the path from the root to the current node, so its length is the depth

LeetCode 1944 note: a decreasing monotonic stack from right to left; everyone popped is visible, and so is the person left on top

LeetCode 2355 note: ending at i, each shelf to the left takes one fewer book until a bottleneck shelf j with too few books; a monotonic stack finds j, and dp[i] = dp[j] + an arithmetic series

LeetCode 316 note: a monotonic stack with last positions; pop a larger top that appears again later, and skip letters already on the stack

LeetCode 726 note: tokenize into elements, numbers, and parentheses; handle parentheses with a stack, with a default multiplier of 1