Pattern · 15 problems
Stack Interview Problems
A stack remembers unfinished business. I push what is waiting and pop when something arrives that settles it.
Free: write the code and run it, no sign-up.
When I reach for it
- Things must close in the reverse order they opened.
- You need the nearest earlier or later item that is bigger or smaller.
- Nesting, undo or parsing shows up in the statement.
Problems, easiest first
2 easy · 9 medium · 4 hard. Open one to read it, then write it in the free editor.
- Valid ParentheseseasyStrings
- Implement Queue using StackseasyDesign
- Reorder ListmediumLinked ListsTwo Pointers
- Car FleetmediumArrays & Hashing
- Min StackmediumDesign
- Valid Parenthesis StringmediumGreedyDynamic Programming
- Daily TemperaturesmediumArrays & Hashing
- Evaluate Reverse Polish NotationmediumArrays & Hashing
- Simplify PathmediumStrings
- Asteroid CollisionmediumArrays & Hashing
- Snack Bin Iterator — Flatten Nested ListsmediumDesign
- Largest Rectangle in HistogramhardArrays & Hashing
- Sliding Window MaximumhardSliding WindowHeaps
- Trapping Rain WaterhardHeapsTwo Pointers
- Basic CalculatorhardStrings
The idea
Ask what is waiting. Brackets wait for a closer. A cold day waits for a warmer one. The stack holds those waiting items, newest on top.
For nearest-greater problems, keep the stack in order. Pop every item the new one beats, answer for each, then push the new item.
pairs = {")": "(", "]": "[", "}": "{"}
stack = []
for ch in text:
if ch in pairs:
if not stack or stack.pop() != pairs[ch]:
return False # closer found no matching opener
else:
stack.append(ch) # opener waits for its closer
return not stack # anything left is unclosedWhere it goes wrong
- Popping from an empty stack.
- Forgetting the leftovers after the loop.
- Pushing values when the answer needs distances. Push indexes instead.
Say this before you type. Say what each item on the stack is waiting for, and what lets you pop it.
Where Coco comes in
Coco is an AI study partner, not a person. She listens while you explain your code, asks what a line does and never hands you the fix. Sign in free to type with Coco, up to a daily limit, and to talk with her for one hour. After that, talking comes with the $79 pack. Anyone can open the editor and run their code.
Questions people ask
- How do I know a stack fits?
- Look for last-in, first-out behavior. The newest unfinished thing is always the next one to settle.
- What is a monotonic stack?
- A stack kept in sorted order by popping items the new one beats. It finds the next greater item for every position in one pass.
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