Parse Variable, Expression, and Application Trees
Reported by candidates from SambaNova Systems's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
The SambaNova Systems OA, reported in March 2022, hands you a string like "[f,(x,y)]" and wants Application(Variable(f),Expression(Variable(x),Variable(y))) back. No algorithm name in the title, but it's a recursive descent parser in disguise. If you've got an invite and 48 hours, this is a string-parsing problem, not a trick problem. Whitespace noise, two bracket types, and nesting up to 500 deep are the only things that can hurt you. StealthCoder sits invisibly on your screen as a safety net if you blank on the parsing structure mid-assessment, but the shape is simple once you see it.
The problem
Parse one valid nested expression into a canonical tree string. An identifier becomes Variable(name). Comma-separated children in parentheses become Expression(child1,...). Comma-separated children in square brackets become Application(child1,...). Whitespace may surround tokens and commas. Each group has at least one child. Return the canonical representation without spaces. Function parseExpressionTree(text: String) → String Examples Example 1 text = "(x,(x,y))" return = "Expression(Variable(x),Expression(Variable(x),Variable(y)))" The nested parentheses produce nested Expression nodes. Example 2 text = "[f,(x,y)]" return = "Application(Variable(f),Expression(Variable(x),Variable(y)))" Square brackets create an Application node. Example 3 text = "value_1" return = "Variable(value_1)" A lone identifier is a Variable. Constraints 1 <= text.length <= 10000. Identifiers contain ASCII letters, digits, and underscore and begin with a letter. The input is syntactically valid, has one root, and nesting depth is at most 500.
Reported by candidates. Source: FastPrep
Pattern and pitfall
The trick is recursive descent with a single index pointer. Write parse(). Skip whitespace. If the char is an open paren, read children separated by commas until the close paren, and wrap them as Expression(...). If it's an open square bracket, do the same and wrap as Application(...). Otherwise read an identifier of letters, digits and underscore, and return Variable(name). The pitfall is whitespace. Skip it before every token and after every child, or commas and closers will trip you. Second pitfall is passing the index by value, so use a class field, a one-element array, or a closure. Depth 500 is fine for recursion in most languages, but a stack works if you're nervous. Build with a list and join, not repeated concatenation. If you freeze on the pointer handling during the live OA, StealthCoder can hand you the working parser as a hedge.
The honest play: practice the pattern, and have StealthCoder ready for the one you didn't see coming.
You can drill Parse Variable, Expression, and Application Trees cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Built for the candidate who saw this exact problem leak two days before his OA and wondered if anyone had a play.
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Parse Variable, Expression, and Application Trees FAQ
How hard is the SambaNova parse expression tree problem really?+
Medium at most. There's no clever algorithm, just careful implementation. The grammar is three cases, the input is guaranteed valid, and there's one root. Most of the difficulty is whitespace handling and keeping your index consistent across recursive calls.
What's the core trick?+
Recursive descent with a shared position index. Each call parses one node, then returns its canonical string. Parens trigger Expression, square brackets trigger Application, and anything else is an identifier turned into Variable(name). Children are joined with commas and no spaces.
Do I need to worry about the depth limit of 500?+
Not really. 500 levels of recursion fits comfortably in default stacks for Python (limit is 1000), Java, and C++. If you're nervous in Python, raise the recursion limit or use an explicit stack. Input length is at most 10000, so linear parsing is fast.
What edge cases should I test before submitting?+
Test a lone identifier like value_1, extra spaces around commas and brackets, deeply nested mixed brackets like [(a,[b,c]),d], and identifiers with digits and underscores. Also confirm the output has no spaces anywhere, since canonical form is strict.
How do I prepare in 48 hours?+
Write a recursive descent parser from scratch twice. Once for this exact grammar, once for something similar like a nested list decoder. Focus on index handling and whitespace skipping. That covers almost everything this problem can throw at you.