Balanced Parentheses Problem Using Stack in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind using a stack to check for balanced parentheses?

Use a stack to keep track of opening parentheses. For every closing parenthesis, check if it matches the last opening one on the stack. If all match and stack is empty at the end, parentheses are balanced.

beginner

Which characters are typically considered in the balanced parentheses problem?

The characters are usually '(', ')', '{', '}', '[' and ']'. Each opening bracket must be matched with the correct closing bracket.

beginner

What happens if you find a closing parenthesis but the stack is empty?

It means there is no matching opening parenthesis for this closing one, so the parentheses are not balanced.

beginner

Why must the stack be empty at the end of the check for balanced parentheses?

If the stack is not empty, it means there are opening parentheses without matching closing ones, so the parentheses are not balanced.

beginner

How do you check if a closing parenthesis matches the last opening parenthesis on the stack?

Compare the closing parenthesis with the opening one on top of the stack. For example, ')' matches '(', '}' matches '{', and ']' matches '['.

What data structure is best suited for checking balanced parentheses?

A Array

B Queue

C Stack

D Linked List

A stack allows last-in, first-out access which is perfect for matching opening and closing parentheses.

What should you do when you encounter an opening parenthesis in the string?

A Pop from the stack

B Ignore it

C Replace it with a closing parenthesis

D Push it onto the stack

Opening parentheses are pushed onto the stack to be matched later with closing parentheses.

If the stack is empty but you find a closing parenthesis, what does it mean?

A Parentheses are balanced

B There is no matching opening parenthesis

C You should push the closing parenthesis

D Ignore the closing parenthesis

A closing parenthesis without a matching opening one means the parentheses are not balanced.

What does it mean if the stack is not empty after processing the entire string?

A There are unmatched opening parentheses

B All parentheses matched

C There are unmatched closing parentheses

D The string is empty

Unmatched opening parentheses remain in the stack, so the parentheses are not balanced.

Which of these pairs correctly matches opening and closing parentheses?

A '{' and '}'

B '(' and '}'

C '[' and '}'

D '(' and ']'

'{' matches with '}', '[' matches with ']', and '(' matches with ')'.

Explain step-by-step how a stack helps to check if parentheses in a string are balanced.

Think about how last opened parentheses must be closed first.

Describe what conditions cause the parentheses to be considered unbalanced when using a stack.

Consider mismatches and leftover openings.