按顺序入栈求出栈的全部可能序列

#include<iostream>
#include<stack>
#include<queue>
#include<algorithm>
using namespace std;

int n;
char a[5000];

void dfs(queue<char> b,int n,stack<int> st,queue<char> out)//example：abc
{
    if(n<=0||(b.empty()&&st.empty()&&out.empty()))
        return;

    
    if (out.size() == n)//7.out3->0;
    {
        while (!out.empty())
        {
            cout << out.front() << " ";
            out.pop();
        }
        cout << endl;
        return;
    }

    stack<int> stcopy = st;
    queue<char> outcopy = out;

    if (!st.empty())//把st的值传给out//2.stc1->0,out1->a 4.stc2->0 out2->ab 6.stc3->0 out3->abc 1.1.stc3->0 b->0
    {
        out.push(st.top());
        st.pop();
        dfs(b, n, st, out);
    }//关键在于stcopy对于上一个st的保存在栈里面。


    if (!b.empty())//如果队列没空把队首的值给1.stcopy//b->bc stc1->a 3.b->c stc2->b 5.b->0 stc3->c
    {
        stcopy.push(b.front());
        b.pop();
        dfs(b, n, stcopy, outcopy);
    }
}
int main()
{
    cin>>n;
    queue<char> b,out;
    stack<int>st;
    for (int i = 0; i < n; i++)
    { 
        cin>> a[i];
        b.push(a[i]);
    }
    dfs(b, n, st, out);
    return 0;

}