Stack

A plain Python list is a stack: append to push, pop() to pop the top.

Core syntax

• Push / pop — stack.append(x) / stack.pop().
• Peek — stack[-1] (guard with if stack).
• Monotonic stack — pop while the top breaks the invariant.

stack = []
for i, h in enumerate(heights):
    while stack and heights[stack[-1]] > h:
        stack.pop()
    stack.append(i)

Watch out

• pop() removes from the end; pop(0) is O(n) — use a deque for a queue.