How do I practice coding and algorithm questions?

Use PracHub's coding console to write, test, and debug your solutions in Python or JavaScript. View hints, test against sample inputs, and compare with official solutions.

What difficulty level is this coding question?

This is a Medium difficulty Coding & Algorithms question, commonly asked during Onsite rounds at Instacart.

What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Instacart during technical interviews.

Implement matrix-indexing and expression resolver

Quick Overview

This question evaluates a candidate's ability to parse structured text files, perform matrix indexing with a nonstandard origin convention, reconstruct data across multiple blocks, and resolve symbolic variable expressions and dependencies for arithmetic evaluation in the Coding & Algorithms domain.

Company: Instacart

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: Medium

Interview Round: Onsite

##### Question Given a text file path whose contents include a single block starting with a line like "[2,4]" followed by rows of equal-length characters (e.g., "S3KDA4" … "RTRYYU"), write code to return the character(s) at each coordinate pair, where [0,0] is the bottom-left character of the block. Follow-up 1 (multiple blocks): each block starts with an integer giving the character’s index in a password, then the "[row,col]" header and the character matrix. Return the reconstructed password across all blocks (e.g., blocks for indices 1 and 0 should output "XK"). Follow-up 2 (multiple passwords): continue reading blocks until an index value repeats; the repeat marks the start of a new password. Return the first complete password only. Expression evaluation: Given a target variable computeExp (e.g., T 3) and a list of assignments such as T1=1, T2=2, T3=T4, T4=T5, T5=T2, compute the numeric value of computeExp by resolving dependencies. Follow-up 1: An assignment may contain exactly one + or – (e.g., T3=T4+T 5). Handle integer arithmetic with one operator. Follow-up 2: If the dependency graph contains a cycle (e.g., T4 depends on T5 and T5 depends on T 4), return "IMPOSSIBLE".

Quick Answer: This question evaluates a candidate's ability to parse structured text files, perform matrix indexing with a nonstandard origin convention, reconstruct data across multiple blocks, and resolve symbolic variable expressions and dependencies for arithmetic evaluation in the Coding & Algorithms domain.

You are given a target variable name (e.g., "T3") and a list of assignment strings of the form "Ti=expr". Each variable name is "T" followed by one or more digits. Each expr is one of: (1) an integer (optionally with a leading minus), (2) a single variable name, or (3) a sum/difference of exactly two terms: term "+" term or term "-" term. A term is either an integer (optionally with a leading minus) or a variable name. Whitespace may appear anywhere. Evaluate the numeric value of the target by resolving dependencies. If the dependency graph contains a cycle, return "IMPOSSIBLE". All referenced variables appear on the left-hand side of some assignment and appear at most once on the left-hand side.

Constraints

1 <= len(assignments) <= 10000
Variable names match regex ^T\d+$
Each variable appears at most once on the left-hand side
Right-hand side expression has at most one binary operator (+ or -)
Terms are either integers (optionally with a leading minus) or variables
All referenced variables are defined in assignments
If a cycle exists, return "IMPOSSIBLE"

Hints

Model dependencies as a directed graph and evaluate using DFS with memoization.
Track node states: unvisited, visiting, and visited to detect cycles.
To parse an expression, strip spaces and split on the first '+' or '-' that is not a leading sign.

Quick Overview