Stack — Last In, First Out

LIFO for matching, undo, and next-greater patterns.

Suno.Stringmeinbracketshain—(),[],{}.Kyaharopenkasahiclosehai,ordersahi?

Listen. String has brackets — (), [], {}. Does every open have a correct close in order?

0:00 / 0:58

Speed

Practice valid-parentheses

When to use this

Parentheses
Monotonic stack
Min-stack

C++ template

stack<char> st;
for (char c : s) {
    if (/* open */) st.push(c);
    else {
        if (st.empty() || st.top() != need) return false;
        st.pop();
    }
}
return st.empty();

Dry run

"()[]{}" → empty stack → true.

Pitfalls

Pop empty
Wrong pair map
Leftover opens

Say this in the interview

Push/pop O(1); full scan O(n).

Problems in this topic (3)

Each opens teach-first (animated when available), then the code panel.