String-Command Calculator

Implement a calculator that consumes a stream of string commands (`ADD 2`, `MULT 3`, ...) and returns the running result. The follow-up nests a calculation inside another (parenthesized sub-expression). Requirements Input: sequence of commands, each a (operator, operand) pair encoded as a string like "ADD 2", "SUB 5", "MULT 3", "DIV 4". Maintain a running accumulator that starts at 0 (or 1 for multiplicative-only sequences; clarify with the interviewer). After each command, return or store the updated accumulator. Follow-up: a command's operand can itself be a nested sub-expression of commands ("calculate this list of commands first, then apply MULT with its result"). Recursive evaluation required. Notes The base version is a one-pass tokenizer + accumulator: split each command on whitespace, dispatch on the operator string, update the accumulator. For the nested follow-up, the natural shape is recursive descent: when a command's operand position contains a sub-expression marker (parentheses or a leading sentinel), recursively evaluate the inner command list and use the result as the operand. Operator precedence is generally not part of the prompt — commands apply left-to-right against the accumulator. Confirm this explicitly; if precedence is required, the round becomes a full expression-evaluator (shunting-yard) instead. Time discipline matters: candidates frequently spend too long on input parsing and run out of time for the nested follow-up. Pre-write a tokenizer helper on the side. Edge cases: empty command list, division by zero, unknown operator (raise or return current accumulator?), nested expression that produces a non-numeric (shouldn't happen in this prompt but worth a guard). Preparation Implement the flat command processor first; verify on a 4-5 command sequence by hand. Add a recursive sub-expression evaluator that consumes the same command grammar. Drill the time-budget: set a 10-minute timer for the flat version so the nested follow-up has room.