I knew the general idea but fumbled the operator precedence part for a bit.
Use a stack to handle operator precedence by evaluating multiplication and division immediately, while deferring addition and subtraction. Parse the string in one pass, maintaining the current number and the last operator, and push results onto the stack. Finally, sum the stack to get the result.
Pro tip: Clarify edge cases upfront, such as division truncation toward zero (e.g., -3/2 = -1) and handling of whitespace, to show attention to detail. Also, mention that you can optimize space by using a variable instead of a stack for the running sum, but a stack is clearer for explaining.
Confirm that the expression is valid, contains only non-negative integers, operators +, -, *, /, and whitespace, and that division truncates toward zero. Discuss potential edge cases like single number, leading/trailing spaces, and large numbers.
Decide to use a stack to store intermediate results. Explain that multiplication and division have higher precedence, so they are evaluated immediately, while addition and subtraction are deferred by pushing numbers onto the stack.
Iterate through the string, building the current number. When an operator is encountered, apply the previous operator to the top of the stack and the current number, then push the result. Update the operator and reset the number.
After the loop, apply the last operator to the final number and the stack. Then sum all elements in the stack to get the final result.
State that time complexity is O(n) and space complexity is O(n) in the worst case. Walk through a few test cases, including expressions with mixed operators and division truncation.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.