Balanced Parentheses Problem Using Stack in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an opening parenthesis onto the stack.

DSA Rust

if ch == '(' {
    stack.[1](ch);
}

Drag options to blanks, or click blank then click option'

Aremove

Bpop

Cpeek

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push will remove an element instead of adding.

Using peek only looks at the top element without adding.

To add an element to the stack, we use the push method.

2fill in blank

medium

Complete the code to check if the stack is empty before popping.

DSA Rust

if !stack.[1]() {
    stack.pop();
}

Drag options to blanks, or click blank then click option'

Ais_empty

Bis_full

Ccontains

Dlen

Attempts:

3 left

💡 Hint

Common Mistakes

Using is_full will not tell if the stack is empty.

Using len returns the number of elements but is not a boolean check.

We check if the stack is empty using is_empty() before popping to avoid errors.

3fill in blank

hard

Fix the error in matching the popped character with the current closing parenthesis.

DSA Rust

if let Some(top) = stack.pop() {
    if top != [1] {
        return false;
    }
}

Drag options to blanks, or click blank then click option'

A'['

B')'

C'('

D'{'

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with the closing parenthesis itself causes logic errors.

Using '[' or '{' does not match the ')' character.

For a closing parenthesis ')', the matching opening is '('. We compare with '('.

4fill in blank

hard

Fill both blanks to correctly match closing brackets with their opening counterparts.

DSA Rust

let matching = match ch {
    ')' => [1],
    ']' => [2],
    _ => ' ',
};

Drag options to blanks, or click blank then click option'

A'('

B'['

C'{'

D')'

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up different types of brackets.

Using closing brackets instead of opening ones.

')' matches '(', and ']' matches '['. These are the correct pairs.

5fill in blank

hard

Fill all three blanks to complete the function that checks balanced parentheses.

DSA Rust

fn is_balanced(s: &str) -> bool {
    let mut stack = Vec::new();
    for ch in s.chars() {
        if ch == [1] || ch == [2] || ch == [3] {
            stack.push(ch);
        } else if ch == ')' || ch == ']' || ch == '}' {
            if stack.is_empty() {
                return false;
            }
            let top = stack.pop().unwrap();
            let matching = match ch {
                ')' => '(',
                ']' => '[',
                '}' => '{',
                _ => ' ',
            };
            if top != matching {
                return false;
            }
        }
    }
    stack.is_empty()
}

Drag options to blanks, or click blank then click option'

A'('

B'['

C'{'

D'<'

Attempts:

3 left

💡 Hint

Common Mistakes

Including invalid characters like '<'.

Missing one or more opening bracket types.

The opening brackets are '(', '[', and '{'. These are pushed onto the stack.

Practice

1. What is the main purpose of using a stack in the Balanced Parentheses Problem?

easy

A. To keep track of opening parentheses and match them with closing ones

B. To count the total number of parentheses in the string

C. To reverse the order of parentheses in the string

D. To sort the parentheses alphabetically

Solution

1. Step 1: Understand the role of stack in parentheses matching

   A stack stores opening parentheses as they appear to check if each closing parenthesis matches the last opened one.

2. Step 2: Explain why other options are incorrect

   Counting, reversing, or sorting parentheses does not help in checking if they are balanced.

3. Final Answer:

   To keep track of opening parentheses and match them with closing ones -> Option A

4. Quick Check:

   Stack tracks openings for matching = B [OK]

Hint: Stack stores openings to match closings quickly [OK]

Common Mistakes:

• Thinking stack counts parentheses instead of matching
• Confusing stack with queue for this problem
• Assuming stack sorts parentheses

2. Which of the following Rust code snippets correctly pushes an opening parenthesis '(' onto a stack named stack?

easy

A. stack.insert('(');

B. stack.push('(');

C. stack.add('(');

D. stack.append('(');

Solution

1. Step 1: Recall Rust Vec method to add elements

   In Rust, Vec uses push() to add elements at the end.

2. Step 2: Verify other methods are invalid for Vec

   add(), insert() require different parameters or do not exist; append() merges vectors, not single elements.

3. Final Answer:

   stack.push('('); -> Option B

4. Quick Check:

   Vec push method = push() [OK]

Hint: Use push() to add items to Rust Vec stack [OK]

Common Mistakes:

• Using append() to add single element
• Confusing insert() with push()
• Using add() which is not a Vec method

3. What will be the output of the following Rust code snippet that checks if parentheses are balanced?

fn is_balanced(s: &str) -> bool {
    let mut stack = Vec::new();
    for ch in s.chars() {
        if ch == '(' {
            stack.push(ch);
        } else if ch == ')' {
            if stack.pop().is_none() {
                return false;
            }
        }
    }
    stack.is_empty()
}

fn main() {
    println!("{}", is_balanced("(()())"));
}

medium

A. false

B. Runtime panic

C. Compilation error

D. true

Solution

1. Step 1: Trace the input string "(()())" character by character

   Process '(', push ['(']; '(', push ['(','(']; ')', pop ['(']; '(', push ['(','(']; ')', pop ['(']; ')', pop []. All match.

2. Step 2: Check final stack state

   Stack is NOT empty after processing all 6 characters because the input string "(()())" has 6 characters but the last character is ')', so the stack ends empty. Wait, let's count carefully:
   Characters: '(' push ['(']
   '(' push ['(','(']
   ')' pop ['(']
   '(' push ['(','(']
   ')' pop ['(']
   ')' pop []
   Stack is empty after processing all characters, so stack.is_empty() returns true.

3. Final Answer:

   true -> Option D

4. Quick Check:

   Matched pairs and empty stack = true [OK]

Hint: Count openings and closings; unmatched means false [OK]

Common Mistakes:

• Mis-counting the number of parentheses
• Ignoring stack emptiness at end
• Confusing pop() returning None with Some

4. Identify the bug in this Rust function that checks balanced parentheses:

fn check_balance(s: &str) -> bool {
    let mut stack = Vec::new();
    for ch in s.chars() {
        if ch == '(' {
            stack.push(ch);
        } else if ch == ')' {
            stack.pop();
        }
    }
    stack.is_empty()
}

medium

A. It uses a wrong data structure instead of Vec

B. It pushes closing parentheses instead of opening

C. It does not check if pop() returns None before popping

D. It returns true even if stack is not empty

Solution

1. Step 1: Analyze pop() usage on empty stack

   The function calls stack.pop() without checking if stack is empty, so popping from empty stack returns None but is ignored.

2. Step 2: Explain consequence of missing check

   This causes false positives when closing parentheses appear without matching openings, as function never returns false early.

3. Final Answer:

   It does not check if pop() returns None before popping -> Option C

4. Quick Check:

   Missing pop() None check causes errors [OK]

Hint: Always check pop() result before proceeding [OK]

Common Mistakes:

• Ignoring pop() returning None on empty stack
• Pushing wrong characters
• Assuming stack emptiness check is enough

5. Given the Rust function below, what will be the output when checking the string "{[(())]}"? The function supports '()', '[]', and '{}' pairs.

fn is_balanced(s: &str) -> bool {
    let mut stack = Vec::new();
    for ch in s.chars() {
        if ch == '(' || ch == '[' || ch == '{' {
            stack.push(ch);
        } else {
            if let Some(top) = stack.pop() {
                if (ch == ')' && top != '(') ||
                   (ch == ']' && top != '[') ||
                   (ch == '}' && top != '{') {
                    return false;
                }
            } else {
                return false;
            }
        }
    }
    stack.is_empty()
}

fn main() {
    println!("{}", is_balanced("{[(())]}"));
}

hard

A. true

B. false

C. Compilation error

D. Runtime panic

Solution

1. Step 1: Trace the string "{[(())]}" step-by-step

   Push '{', '[', '(', '('. Then pop for ')', ')', ']', '}'. Each closing matches the last opening correctly.

2. Step 2: Check stack emptiness after processing

   All pairs matched, stack is empty, so function returns true.

3. Final Answer:

   true -> Option A

4. Quick Check:

   All pairs matched and stack empty = true [OK]

Hint: Match each closing with last opening in stack [OK]

Common Mistakes:

• Not handling multiple bracket types
• Ignoring order of matching pairs
• Forgetting to check stack empty at end