Min Insertion to Balance a Parentheses String
Reported by candidates from IBM's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
IBM threw a parentheses balancing problem at candidates in July 2024, and it's the kind of thing that trips people up if you haven't seen the pattern before. You're given a string of unbalanced parentheses and need to find the minimum number of insertions to make it valid. It's a string manipulation problem disguised as a greedy algorithm. The trick is that you don't need dynamic programming here, even though the problem feels like it might. StealthCoder will catch you if you freeze on the approach.
Pattern and pitfall
The core pattern is greedy with a counter. Walk through the string left to right and track open parentheses. When you hit a close paren, match it if you have an open one available, otherwise count it as an insertion. After the pass, any remaining unmatched open parens also need closes. The insight: you're not rearranging or searching backwards. You're just counting mismatches in a single pass. The common pitfall is overthinking it as a DP problem or trying to find an optimal insertion position. IBM's version likely has straightforward test cases where the greedy approach surfaces instantly if you recognize the pattern. If you blank during the live OA, StealthCoder will feed you the counter-based solution so you can code it cleanly.
Drill it cold or hedge it with StealthCoder. Either way, don't walk into the OA hoping you remember the trick.
You can drill Min Insertion to Balance a Parentheses String 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. Made for the candidate who got the OA invite this morning and has 72 hours, not six months.
Get StealthCoderRelated leaked OAs
This OA pattern shows up on LeetCode as minimum insertions to balance a parentheses string. If you have time before the OA, drill that.
You've seen the question.
Make sure you actually pass IBM's OA.
IBM reuses patterns across OAs. Made for the candidate who got the OA invite this morning and has 72 hours, not six months. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Min Insertion to Balance a Parentheses String FAQ
Is this problem asking me to output the actual insertions or just count them?+
The problem title says 'min insertion', so you're counting. You don't need to reconstruct the string or list positions. Just return an integer representing the minimum number of characters to insert to balance the string.
Do I need to handle nested or mixed bracket types?+
The problem mentions 'parentheses string', so you're only dealing with '(' and ')'. No curly braces, square brackets, or nested types. Keep it simple.
What's the time complexity I should aim for?+
Single pass through the string. O(n) time, O(1) space. You're just counting, not building a data structure. If your solution is doing anything else, you've overengineered it.
Is this still a common pattern in 2024 OAs, or is IBM moving on to harder stuff?+
String and greedy problems haven't gone anywhere. IBM still tests fundamentals. This problem is straightforward once you see it, which is exactly why it's useful as a filtering question.
How do I prepare in 48 hours if I'm weak on greedy string problems?+
Do three simple problems: reverse a string, valid parentheses check, and a greedy character replacement problem. Focus on the pattern, not the volume. Speed comes from recognizing the shape, not from drilling 50 variations.