← Walmart Labs Interview Insights
I knew to use a stack immediately, which helped.
Use a stack to track opening brackets. Iterate through the string once: push opening brackets onto the stack; for closing brackets, check if the stack is non-empty and the top matches the corresponding opening bracket, then pop. At the end, the stack must be empty for the brackets to be balanced.
Pro tip: Explicitly state the time and space complexity: O(n) time and O(n) space in the worst case. Also, mention that early termination (e.g., returning false as soon as a mismatch is found) can optimize average-case performance.
Confirm that the string may contain other characters besides brackets and that we only care about bracket matching. Ask if the input can be empty or null.
Explain that a stack is ideal because brackets must be closed in the reverse order they were opened (LIFO).
Describe the single-pass approach: iterate through each character; if it's an opening bracket, push it; if it's a closing bracket, check if the stack is empty or the top doesn't match, then return false; otherwise pop. After the loop, return true only if the stack is empty.
State that the algorithm runs in O(n) time and uses O(n) space in the worst case (e.g., all opening brackets).
Walk through a few test cases: valid string like '()[]{}', invalid like '([)]', and edge cases like empty string or single bracket.
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