ITPEC FE Subject B April 2024 Question 13

Balanced brackets check using a stack.

The program checks whether a string of brackets is balanced. It iterates through each character:
- If the character is an opening bracket ((, {, [), it is pushed onto the stack.
- If it is a closing bracket, the top of the stack is popped and its expected closing bracket is looked up via get_closing_bracket.

Blank A: `≠ c`
After popping the stack and finding the expected closing bracket, we compare it with the current character c. If they do not match (≠ c), the brackets are mismatched → return false.

Blank B: `stack.isEmpty()`
After processing all characters, we check whether the stack is empty. If it is, every opening bracket had a match → return true. If not, some brackets were never closed → return false.

Blank C: `chars[2]`
get_closing_bracket finds the bracket pair where chars[1] (the opening bracket) matches the input. It returns chars[2] — the corresponding closing bracket. (Arrays are 1-indexed in this pseudocode.)

Why not others:
- (a–d) all use = c for A, which inverts the mismatch check — would return false when brackets do match.
- (e) A is correct (≠ c), but C = chars[1] returns the opening bracket instead of the closing one.
- (f) A and C are correct, but B = not stack.isEmpty() inverts the final check — returns true when unmatched brackets remain.
- (g) A and B are correct, but C = chars[1] returns the opening bracket instead of closing.

Key rule: In balanced bracket problems, after processing all characters, stack.isEmpty() must return true for the expression to be balanced. With 1-indexed arrays, chars[2] is the closing bracket in each pair.

AI-generated — may contain errors