3-3.cpp

#include <iostream>
#include <vector>
#include <stack>
using namespace std;

class SetOfStacks //stacks managed by vector
{
private:
    int capacity;
    vector<stack<int>> vs;
    stack<int> get_last() {
        //return vs[vs.size()-1];
        return vs.at(vs.size()-1);
    }
public:
    SetOfStacks(int cap) {capacity = cap;}
    
    void push(int v) {
        //cout << vs.size() << endl;
        
        //if no stack, push new stack
        if (vs.size() == 0) {
            cout << "new vector" << endl;
            stack<int> s;
            vs.push_back(s);
        }
        //if current stack's full, push new stack
        //imagine maimum is 3
        else if (get_last().size() == capacity) {
            cout << "add new stack" << endl;
            stack<int> s;
            vs.push_back(s);
        }
        
        stack<int> temp = get_last();
        temp.push(v);
        vs.pop_back();
        vs.push_back(temp);
    }
    
    void pop() {
        if (vs.size() == 0) {
            return;
        }
        //if last element of the stack, remove the stack from vector
        else if (get_last().size() == 1) {
            vs.pop_back();
        }
        //else, normal pop
        else {
            stack<int> temp = get_last();
            temp.pop();
            vs.pop_back();
            vs.push_back(temp);
        }       
    }
    
    int top() {
        if (vs.size() != 0 && get_last().size() != 0) {
            return get_last().top();
        } else {
            return NULL;
        }
    }
    
    int stack_num() {
        return vs.size();
    }
};
int main(int argc, char *argv[]) {
    //test case
    SetOfStacks ss(3);
    ss.push(1);
    ss.push(2);
    ss.push(3);
    ss.push(4);
    cout << ss.stack_num() << endl;;
    
}