LIFO converts to FIFO using a recursive function

gistfile1.cpp

#include <list>
#include <cstdio>

using namespace std;

class LIFO {
public:
   void push_back(int a) {
       mList.push_back(a);
    }

    int pop_back() {
        int a = mList.back();
        mList.pop_back();
        return a;
    }

    int size() {
        return mList.size();
    }

private:
    list<int> mList;
};

class FIFO {
public:
    void push_back(int a) {
        mLIFO.push_back(a);
    }

    int pop_first() {
        int a = mLIFO.pop_back();
        if (mLIFO.size() == 0)
            return a;

        int b = pop_first();
        mLIFO.push_back(a);
        return b;
    }

private:
    LIFO mLIFO;
};

int main() {
    FIFO fifo;
    
    for (int i = 0; i < 10; ++i)
        fifo.push_back(i);

    for (int i = 0; i < 10; ++i)
        printf("%d\n", fifo.pop_first());

    return 0;
}