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26 worked problems

Stack

LIFO bookkeeping for matching and order. The most recent unresolved item must be handled first, or you need the next greater or smaller boundary.

Recognition signal

When should you think of Stack?

The most recent unresolved item must be handled first, or you need the next greater or smaller boundary.

Core invariant

What must stay true?

The stack contains unresolved items in the exact order or monotonic shape required.

Practice in a deliberate order

Starter

Build the template

  1. #20 Valid ParenthesesEasy
  2. GFG Implement Stack using ArraysEasy
  3. GFG Implement Queue using ArraysEasy
Core

Recognize variations

  1. #71 Simplify PathMedium
  2. #150 Evaluate Reverse Polish NotationMedium
  3. #155 Min StackMedium
  4. #225 Implement Stack using QueueMedium
  5. #227 Basic Calculator IIMedium
  6. #232 Implement Queue using StackMedium
Stretch

Combine ideas under pressure

  1. #32 Longest Valid ParenthesesHard
  2. #84 Largest Rectangle in HistogramHard
  3. #85 Maximal RectangleHard
  4. #224 Basic CalculatorHard

What to learn in this cluster

  1. 01Matching and parsing
  2. 02Monotonic stacks
  3. 03Expression and state simulation

All Stack problems

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