Roadmap › Stage 4: Stacks › Pattern 19 of 53
DSA pattern guide

Stack — Expressions

Evaluate or decode an expression with operators, brackets or nesting.

Stage 4 · Stacks4 problems3 Medium · 1 Hard

Use it when

Evaluate or decode an expression with operators, brackets or nesting.

Key idea

Push numbers and operators (or partial strings). On a closing bracket or lower-precedence operator, pop and combine.

Practice problems, in order

  1. Basic Calculator IIMedium
  2. Evaluate Reverse Polish NotationMedium
  3. Decode StringMedium
  4. Basic Calculator IHard

How to practise this pattern

  1. Read the problem and look for the signal above before writing any code.
  2. Say the brute force out loud, with its time complexity, then ask what work it repeats.
  3. Apply the key idea and write the optimised version yourself.
  4. If you're stuck for more than 10 minutes, take one small hint, not the full solution.
  5. Move to the next problem only when you can explain why this pattern fits.

Stuck on one of these?

The AlgoSolveo extension (free, launching soon) sits beside the LeetCode problem and gives the smallest hint that gets you moving, never the full answer. Solved problems tick off on your roadmap.

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