Software Engineer · Onsite · Coding

OpenAI — Software Engineer ✅ Passed

Level: Staff-Level

Round: Onsite · Type: Coding · Difficulty: 7/10 · Duration: 60 min · Interviewer: Neutral

Topics: Recursion, Hash Table

Location: San Francisco Bay Area

Interview date: 2026-01-04

Summary

The question involves implementing the string representation of types in a toy language that supports primitives, generics, and tuples. I needed to implement Node.toString() for different types and Function.toString() for function types, adhering to specific formatting rules.

Details

The coding question I got was:

You are working with a toy language grammar that supports three kinds of types: primitives, generics, and tuples. This system is used for defining function types, including parameter and return types.

• Primitive types: "int", "char", "float"
• Generic types: Strings following the pattern "T" plus a digit, such as "T1", "T2". A generic can stand for any type, including a tuple.
• Tuple types: Represented as a list of types, for example ["int", "T1", "char"]. Tuples may be nested, such as ["int", "char", ["int", "T1"]]. A Node represents either a primitive, a generic, or a tuple.

A Function is described by its parameter list, params, and its return type, returnType. Both parameters and return type are represented as Node objects.

Your task is to implement the string representation of these types, following the exact format below:

• Node.toString() returns a string for the type:

  • For a primitive or generic, return its value directly, such as "int" or "T2".
  • For a tuple, return its children in square brackets, separated by commas, such as "[int,T1,char]". There should be no extra spaces between each child. Nested tuples must be handled recursively.

• Function.toString() returns the function type as "[<param1>,<param2>,...] -> <returnType>", where parameters and return type are rendered using Node.toString().

Constraints:

• All type names are valid.
• The params list and tuple children may be empty.
• Nesting is allowed to any depth within tuples.

Example 1: Input: Node representing primitive type "int". Output: "int"

Example 2: Input: Node representing tuple ["int", "T1", "char"]. Output: "[int,T1,char]"

Example 3:

Input: Function with params ["int", "T1", ["int", "T2"]] and return type "T1" Output: "[int,T1,[int,T2]] -> T1"

My approach:

1. First, I analyzed the requirements and constraints of the problem.
2. Then, I implemented the Node.toString() method by checking the type of the node (primitive, generic, or tuple).
3. For tuples, I recursively called Node.toString() on each child and constructed the string representation.
4. I implemented the Function.toString() method by using Node.toString() to represent the parameters and return type, and assembling the final string.

Key insights:

• Properly handling recursion for nested tuples is crucial.
• Paying close attention to the formatting requirements (e.g., no extra spaces) is important.
• Understanding the different types of nodes (primitive, generic, tuple) is essential for the correct implementation.