The precedence handling is where people mess up.
Clarify the problem constraints and edge cases, then propose a two-pass stack-based approach that handles operator precedence and left-to-right evaluation. Walk through an example to demonstrate correctness, and discuss time/space complexity and potential trade-offs.
Pro tip: Mention that you can avoid using a stack by first tokenizing and then evaluating with two passes, but highlight that the stack approach is more extensible for adding parentheses or more operators. Also, explicitly handle division truncation toward zero, especially for negative intermediate results (though inputs are non-negative, intermediate results can be negative).
Ask about input format, possible edge cases (e.g., empty string, single number, multiple spaces, division by zero), and confirm that division truncates toward zero. Discuss how to handle negative intermediate results.
Propose a stack-based solution: parse the string, maintain a current number and a last operator, and use a stack to handle precedence. Alternatively, mention a two-pass approach (first handle * and /, then + and -) but note its limitations.
Explain step-by-step: iterate through characters, build multi-digit numbers, when an operator is encountered, apply the previous operator to the current number and the top of the stack (or push/pop accordingly), then update the operator. At the end, sum the stack.
State that time complexity is O(n) and space complexity is O(n) for the stack. Discuss trade-offs: stack uses extra space but simplifies precedence; two-pass avoids stack but requires more parsing logic and is less extensible.
Walk through a sample expression like '3+2*2' to show how the stack evolves, and mention testing edge cases like '14-3/2' (truncation) and ' 3/2 ' (spaces).
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