Remove K-Balanced Substrings

Remove K-Balanced Substrings

What is this problem about?

The Remove K-Balanced Substrings interview question asks you to remove all substrings of the form [...k...] where [ and ] are matched brackets and k is a specific character contained within them, then return the resulting string. This problem simulates the process of repeatedly detecting and collapsing balanced bracket groups from innermost to outermost, much like evaluating expressions in a compiler or interpreter.

Why is this asked in interviews?

This problem is asked at Deloitte and Bloomberg because it tests the ability to use a stack for balanced delimiter matching — a core concept in parsing, syntax checking, and expression evaluation. It is a practical simulation problem that rewards candidates who understand when to use a stack to handle nested or sequential bracket structures and can track the enclosed content efficiently.

Algorithmic pattern used

The pattern is stack simulation. Process the string character by character. Push each character onto a stack. When you encounter ], pop characters from the stack until you find [. Examine the popped characters: if the enclosed content contains the target character k, discard the entire group. Otherwise, push the group back (i.e., restore it as it represents a non-matching bracket group). At the end, reconstruct the string from the stack.

Example explanation

String: "leetcode", k = 'e', brackets are [ and ]. Consider "a[b]c[de]f" with k = 'd':

• Push a, [, b, then ] is encountered → pop until [, content is b. b does not contain d → push back [b].
• Push c, [, d, e, then ] → pop until [, content is de. Contains d → discard entire group.
• Push f.

Result: "a[b]cf".

Common mistakes candidates make

• Forgetting to check the content between brackets before deciding to remove — not every bracketed group should be removed.
• Rebuilding the result from the stack in the wrong order — the stack is LIFO, so reverse it when reconstructing.
• Not handling nested brackets correctly — the stack approach naturally handles nesting layer by layer.
• Treating non-bracket characters incorrectly when they appear adjacent to brackets.

Interview preparation tip

For the Remove K-Balanced Substrings coding problem, the stack and string simulation interview pattern is the right tool. Before coding, mentally trace through a small example with nested brackets to verify your stack logic. Bloomberg interviewers often look for clean stack-based solutions for this class of problems. Practice the general pattern: push until closing delimiter, pop and inspect content, decide to keep or discard.