3-2.cpp

#include <iostream>
#include <stack>
using namespace std;
class MyStack
{
private:
    stack<int> s;
    stack<int> min;
public:
    void push(int v) {
        s.push(v);
        if (min.empty() || v <= min.top()) { //enter if first satisfy
            min.push(v);
        }
    }
    void pop() {
        if (s.top() == min.top()) {
            min.pop();
        }
        s.pop();
        
    }
    int top() {
        return s.top();
    }
    int get_min() {
        return min.top();
    }
};
int main(int argc, char *argv[]) {
    //test case
    MyStack ms;
    ms.push(2);
    ms.push(3);
    ms.push(1);
    cout << ms.get_min() << endl;
    
    ms.pop();
    cout << ms.get_min() << endl;
    ms.pop();
    cout << ms.get_min() << endl;
    
}