Stack

Monotonic stack on a circular array

Monotonic decreasing stack over nums2 to build next-greater map; answer queries for nums1.

LeetCode 2487 note: the kept nodes are non-increasing left to right; keep a monotonic stack, pop anything smaller than the new node, then link what remains

LeetCode 2000 note: find the first index i of ch, reverse word[:i + 1] and append the rest; return the word unchanged if ch is absent

LeetCode 735 note: scan left to right with a stack; only a right-moving top meeting a left-moving asteroid collides, the smaller explodes and equal sizes both do

LeetCode 402 note: a monotonically increasing stack; while deletions remain, pop any top larger than the current digit, then strip leading zeros

LeetCode 227 note: read whole numbers, and at each operator apply the previous one: + and - push the number (negated for -), * and / combine with the top

LeetCode 143 note: find the middle, reverse the second half, and merge the two alternately, cutting the first half’s tail

LeetCode 42 note: each cell holds min(tallest on the left, tallest on the right) - its own height; use prefix and suffix maxima or two pointers

LeetCode 71 note: split on /, skip empty pieces and ., pop on .. (never above root), push other names, and join with /

LeetCode 1047 note: if the stack top equals the current character both vanish, otherwise push it; after a removal the new top may cancel with the next character

LeetCode 1021 note: track depth instead of a stack; the ‘(’ raising depth from 0 to 1 and the ‘)’ lowering it from 1 to 0 are outermost, everything else stays