Roblox interview question Most Frequent Call Stack – Debugging Trace Analysis

Your team is debugging a program’s performance. A tracing tool records all function calls and returns, producing a sequence of events.
Your task is to analyze these raw traces and determine which call stack occurs most frequently.
Finding the most frequent call stack will help identify where the main slowdown may be occurring.

Input

• An array of strings, each representing a traced function call or return.
• Function calls are prefixed by "->" (e.g. -> main).
• Function returns are prefixed by "<-" (e.g. <- main).

Output

• Return the call stack string (functions joined by "->") that appears most frequently.
• If there’s a tie, return the one that first reached the top frequency.

Call Stack Definition

• A call stack represents the sequence of active function calls at any time.
• Example:
  If main() calls handleEvents(), which calls handleClickEvent(),
  then the current call stack is
  main->handleEvents->handleClickEvent.
• Returning from a function simply pops the top frame—it does not create a new stack identity.

Example

traces = [
    "-> main",
    "-> handleEvents",
    "-> handleClickEvent",
    "<- handleClickEvent",
    "<- handleEvents",
    "-> handleEvents",
    "<- handleEvents",
    "<- main"
]

Output:
"main->handleEvents->handleClickEvent"
Because this stack occurred twice, which is the highest frequency.

Constraints

• The trace is guaranteed to be well-formed: every function call has a corresponding return.

This problem tests understanding of stack simulation and frequency counting under nested call conditions.
The key idea is to maintain a dynamic stack that records the current function calls.
Each time a new call happens, join the current stack into a string (e.g., "main->handleEvents->click") and count its occurrences.
When a return occurs, pop the function from the stack.

Algorithm Outline

1. Initialize an empty stack and a frequency dict.
2. For each trace:
   • If it starts with "->", push the function name to the stack.
     • Convert the current stack into a string joined by "->" and increment its frequency.
   • If it starts with "<-", pop the top function from the stack.
3. Return the stack string with the highest frequency; if tied, return the first one that reached that frequency.

Complexity

• Time: O(n) — single pass through traces
• Space: O(n) — to store active stack and frequency map

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