Valid Parentheses

Stack · Last opened, first closed · Easy · about 15 min

Why it matters

Stacks model nested work: brackets, function calls, undo, and expression parsing.

How it connects

Arrays can also behave like a stack when the newest unfinished thing matters most.

Try first

When you see a closing bracket, which opening bracket is allowed to match it?

Interview cue

The stack preserves nesting order, and each character is pushed and popped at most once.