Software Engineer · Full Journey · Coding

Roblox — Software Engineer ✅ Passed

Level: Senior-Level

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

Topics: Data Structures, Algorithms, Call Chain Analysis

Location: San Francisco Bay Area

Interview date: 2026-01-20

Summary

Interview Rounds Overview

• Round 1: Coding

Details

I was asked the following coding question:

A software system logs its execution traces as a sequence of program events, where each event records either a function call or a function return. Each entry in the input list traces is one of:

• "-> function_name": indicates that function_name has been called and is added to the call chain.
• "<- function_name": indicates that function_name has returned and is removed from the top of the call chain.

At any point, the call chain represents the list of active function calls, ordered from the first called (bottom) to the most recent (top). Every time a function is called (i.e., for every "-> function_name" in traces), record the current call chain immediately after the call.

Your task is to find the most frequently occurring call chain across the execution. The call chain should be output as a list of function names, from first called to last called, joined by "->".

If multiple call chains appear with the same maximum frequency, return the one with the greatest depth (most functions on the chain). And the result is guaranteed to be unique after applying these rules.

Return a string array of length 2, where:

• The first element is a single string representing the call chain.
• The second element is a string indicating the number of times this chain occurs.

Constraints:

`

1. 1 ≤ traces.length ≤ 10^5
2. Each element in traces is a string in the format "-> function_name" or "<- function_name".
3. function_name contains only alphanumeric characters and underscores.
4. The trace sequence is valid (no unmatched calls or returns). `

Example 1:

Input: traces = ["-> main", "-> onButtonPress", "-> validateUserInput", "<- validateUserInput", "-> validateUserInput", "<- validateUserInput", "<- onButtonPress", "-> onKeyboardInput", "<- onKeyboardInput", "<- main"]

Output: ["main -> onButtonPress -> validateUserInput", "2"]

Explanation: The function call traces are shown below:

-> main -> onButtonPress -> validateUserInput <- validateUserInput -> validateUserInput <- validateUserInput <- onButtonPress -> onKeyboardInput <- onKeyboardInput <- main The call chain "main -> onButtonPress -> validateUserInput" appears twice, which is more than any other call chains.

Example 2:

Input: traces = ["-> func1", "-> func2", "<- func2", "-> func2", "<- func2", "<- func1"]

Output: ["func1 -> func2", "2"]

Example 3:

Input: traces = ["-> level1", "-> level2", "-> level3", "-> level4", "<- level4", "<- level3", "<- level2", "-> level2", "-> level3", "-> level4", "<- level4", "<- level3", "<- level2", "<- level1"]

Output: ["level1 -> level2 -> level3 -> level4", "2"]