Stack with const time get_min() function

min_stack.cpp

#include <cstdlib>
#include <iostream>
#include <stack>

class CustomStack {

public:
    void push(int num);
    void pop();
    int top();
    int get_min();

private:
    std::stack<int> m_elem;
    int m_minval;
};

void CustomStack::push(int num)
{
    if (m_elem.empty()) {
        m_elem.push(0);
        m_minval = num;
    }
    else {
        m_elem.push(num - m_minval);
        if (num < m_minval) {
            /* updating min val */
            m_minval = num;
        }
    }
}

void CustomStack::pop()
{
    int delta = m_elem.top();
    m_elem.pop();
    if (delta < 0) {
        /* IMP: revert to previous minimum */
        m_minval -= delta;
    }
}

int CustomStack::top()
{
    if (m_elem.top() > 0) {
        /* return the normalized value */
        return m_minval + m_elem.top();
    }
    else {
        return m_minval;
    }
}

int CustomStack::get_min()
{
    return m_minval;
}

int main()
{
    srand(time(NULL));
    /* Lets create a stack and push some random values */
    CustomStack s;
    for (int i = 0; i < 10; i++) {
        int num = rand() % 100;
        std::cout << "Inserting :" << num << std::endl;
        s.push(num);
    }
    std::cout << std::endl;
    std::cout << "Min is " << s.get_min() << std::endl;
    s.pop();
    s.pop();
    s.pop();
    s.pop();
    std::cout << "After 4 pop(s), min is " << s.get_min() << std::endl;
    return 0;
}