SimulationStack.cpp

/*
Problem: Simulation Stack

Description
Perform a sequence of operations over a stack, each element is an integer:
PUSH v: push a value v into the stack
POP: remove an element out of the stack and print this element to stdout (print NULL if the stack is empty)

Input
Each line contains a command (operration) of type 
PUSH  v
POP
Output
Write the results of POP operations (each result is written in a line)
Example

Input
PUSH 1
PUSH 2
PUSH 3
POP
POP
PUSH 4
PUSH 5
POP
#

Output
3
2
5
*/

//CPP 
#include <bits/stdc++.h> 
using namespace std;
int main() 
{ 
    ios_base::sync_with_stdio(false); cin.tie(0);
    string cmd;
    int numb;
    stack<int> st;
    do {
        cin >> cmd;
        if (cmd == "#") {
            continue;
        }
        if (cmd == "PUSH") {
            cin >> numb;
            st.push(numb);
        }
        if (cmd == "POP") {
            if (st.empty()) {
                cout << "NULL" << endl;
            }
            else {
                int s = st.top();
                st.pop();
                cout << s << endl;
            }
        }
    } while (cmd != "#");
    return 0;
}