# Interleave the first half of the queue with second half

Given a queue of integers of even length, rearrange the elements by interleaving the first half of the queue with the second half of the queue.

Only a stack can be used as an auxiliary space.

Examples:

```Input :  1 2 3 4
Output : 1 3 2 4

Input : 11 12 13 14 15 16 17 18 19 20
Output : 11 16 12 17 13 18 14 19 15 20
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Following are the steps to solve the problem:
1.Push the first half elements of queue to stack.
2.Enqueue back the stack elements.
3.Dequeue the first half elements of the queue and enqueue them back.
4.Again push the first half elements into the stack.
5.Interleave the elements of queue and stack.

```// C++ program to interleave the first half of the queue
// with the second half
#include <bits/stdc++.h>
using namespace std;

// Function to interleave the queue
void interLeaveQueue(queue<int>& q)
{
// To check the even number of elements
if (q.size() % 2 != 0)
cout << "Input even number of integers." << endl;

// Initialize an empty stack of int type
stack<int> s;
int halfSize = q.size() / 2;

// Push first half elements into the stack
// queue:16 17 18 19 20, stack: 15(T) 14 13 12 11
for (int i = 0; i < halfSize; i++) {
s.push(q.front());
q.pop();
}

// enqueue back the stack elements
// queue: 16 17 18 19 20 15 14 13 12 11
while (!s.empty()) {
q.push(s.top());
s.pop();
}

// dequeue the first half elements of queue
// and enqueue them back
// queue: 15 14 13 12 11 16 17 18 19 20
for (int i = 0; i < halfSize; i++) {
q.push(q.front());
q.pop();
}

// Again push the first half elements into the stack
// queue: 16 17 18 19 20, stack: 11(T) 12 13 14 15
for (int i = 0; i < halfSize; i++) {
s.push(q.front());
q.pop();
}

// interleave the elements of queue and stack
// queue: 11 16 12 17 13 18 14 19 15 20
while (!s.empty()) {
q.push(s.top());
s.pop();
q.push(q.front());
q.pop();
}
}

// Driver program to test above function
int main()
{
queue<int> q;
q.push(11);
q.push(12);
q.push(13);
q.push(14);
q.push(15);
q.push(16);
q.push(17);
q.push(18);
q.push(19);
q.push(20);
interLeaveQueue(q);
int length = q.size();
for (int i = 0; i < length; i++) {
cout << q.front() << " ";
q.pop();
}
return 0;
}
```

Output:

```11 16 12 17 13 18 14 19 15 20
```

Time complexity: O(n).
Auxiliary Space : O(n).

This article is contributed by Prakriti Gupta. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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