Copy elements from queue to stack - c++

Question

I want to write a code that copies queue elements in to a stack, and this copied elements should be sorted in the stack.
i've written the code below:
#include <bits/stdc++.h>
using namespace std;
int main(){
int n,x;
queue<int> q;
stack<int> s;
cin >> n;
for(int i=0;i<n;i++){
cin >> x;
q.push(x);
}
while(!q.empty()){
if(s.empty()){
s.push(q.front());
q.pop();
}
else{
if(q.front()>=s.top()){
s.push(q.front());
q.pop();
}
else{
while(q.front()<s.top() && !s.empty()){
q.push(s.top());
s.pop();
}
}
}
}
while(!s.empty()){
cout << s.top() << " ";
s.pop();
}
return 0;
}
but for some test cases like:3 1 2 3,the code does not seem to work.
please help me to figure out the problem with my code.

Answer 1

Your code seems to have a logic error in your inner while loop. You test s.top() before you test for s.empty(), which is the wrong order. s.top() is invalid if s.empty() is true. However, the exercise can be resolved relatively easily as described below.
You can retrieve the underlying container of a container adapter using the following helper:
template <class ADAPTER>
typename ADAPTER::container_type & get_container (ADAPTER &a)
{
struct hack : ADAPTER {
static typename ADAPTER::container_type & get (ADAPTER &a) {
return a.*&hack::c;
}
};
return hack::get(a);
}
Then, after populating the queue, you can copy the contents directly to the stack's underlying container, and sort it.
auto &qc = get_container(q);
auto &sc = get_container(s);
sc.assign(qc.begin(), qc.end());
std::sort(sc.begin(), sc.end());

Answer 2

You have to do what is essentially a selection sort: go through the queue N times, each time selecting the largest element to push onto the stack. The general idea is:
queue = queue containing items
stack = new empty stack
while queue.count > 0
// push the first item from the queue onto the stack
stack.push(queue.pop())
count = queue.count
// for each item remaining in the queue
for (i = 0; i < queue.count; ++i)
// if the item from the queue is larger than what's on the stack,
// then remove the item from the stack and put it back in
// the queue. And put the item from the queue onto the stack.
if (queue.peek() > stack.peek())
queue.push(stack.pop())
stack.push(queue.pop())