# 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++

 `// C++ program to interleave the first half of the queue ` `// with the second half ` `#include ` `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; ` `} `

## Java

 `// Java program to interleave ` `// the first half of the queue ` `// with the second half ` `import` `java.util.*; ` ` `  `class` `GFG  ` `{ ` ` `  `// Function to interleave the queue ` `static` `void` `interLeaveQueue(Queueq) ` `{ ` `    ``// To check the even number of elements ` `    ``if` `(q.size() % ``2` `!= ``0``) ` `        ``System.out.println(``"Input even number of integers."` `); ` ` `  `    ``// Initialize an empty stack of int type ` `    ``Stack s = ``new` `Stack<>(); ` `    ``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.peek()); ` `        ``q.poll(); ` `    ``} ` ` `  `    ``// enqueue back the stack elements ` `    ``// queue: 16 17 18 19 20 15 14 13 12 11 ` `    ``while` `(!s.empty())  ` `    ``{ ` `        ``q.add(s.peek()); ` `        ``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.add(q.peek()); ` `        ``q.poll(); ` `    ``} ` ` `  `    ``// 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.peek()); ` `        ``q.poll(); ` `    ``} ` ` `  `    ``// interleave the elements of queue and stack ` `    ``// queue: 11 16 12 17 13 18 14 19 15 20 ` `    ``while` `(!s.empty()) ` `    ``{ ` `        ``q.add(s.peek()); ` `        ``s.pop(); ` `        ``q.add(q.peek()); ` `        ``q.poll(); ` `    ``} ` `} ` ` `  `// Driver code ` `public` `static` `void` `main(String[] args)  ` `{ ` `    ``Queue q = ``new` `java.util.LinkedList<>(); ` `    ``q.add(``11``); ` `    ``q.add(``12``); ` `    ``q.add(``13``); ` `    ``q.add(``14``); ` `    ``q.add(``15``); ` `    ``q.add(``16``); ` `    ``q.add(``17``); ` `    ``q.add(``18``); ` `    ``q.add(``19``); ` `    ``q.add(``20``); ` `    ``interLeaveQueue(q); ` `    ``int` `length = q.size(); ` `    ``for` `(``int` `i = ``0``; i < length; i++)  ` `    ``{ ` `        ``System.out.print(q.peek() + ``" "``); ` `        ``q.poll(); ` `    ``} ` `} ` `} ` ` `  `// This code contributed by Rajput-Ji `

## Python3

 `# Python3 program to interleave the first  ` `# half of the queue with the second half  ` `from` `queue ``import` `Queue  ` ` `  `# Function to interleave the queue  ` `def` `interLeaveQueue(q): ` `     `  `    ``# To check the even number of elements  ` `    ``if` `(q.qsize() ``%` `2` `!``=` `0``):  ` `        ``print``(``"Input even number of integers."``) ` ` `  `    ``# Initialize an empty stack of int type  ` `    ``s ``=` `[] ` `    ``halfSize ``=` `int``(q.qsize() ``/` `2``)  ` ` `  `    ``# put first half elements into  ` `    ``# the stack queue:16 17 18 19 20,  ` `    ``# stack: 15(T) 14 13 12 11 ` `    ``for` `i ``in` `range``(halfSize): ` `        ``s.append(q.queue[``0``])  ` `        ``q.get() ` ` `  `    ``# enqueue back the stack elements  ` `    ``# queue: 16 17 18 19 20 15 14 13 12 11  ` `    ``while` `len``(s) !``=` `0``:  ` `        ``q.put(s[``-``1``])  ` `        ``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` `i ``in` `range``(halfSize): ` `        ``q.put(q.queue[``0``])  ` `        ``q.get() ` ` `  `    ``# Again put the first half elements into  ` `    ``# the stack queue: 16 17 18 19 20, ` `    ``# stack: 11(T) 12 13 14 15  ` `    ``for` `i ``in` `range``(halfSize): ` `        ``s.append(q.queue[``0``])  ` `        ``q.get() ` ` `  `    ``# interleave the elements of queue and stack  ` `    ``# queue: 11 16 12 17 13 18 14 19 15 20  ` `    ``while` `len``(s) !``=` `0``:  ` `        ``q.put(s[``-``1``])  ` `        ``s.pop()  ` `        ``q.put(q.queue[``0``])  ` `        ``q.get() ` ` `  `# Driver Code ` `if` `__name__ ``=``=` `'__main__'``: ` `    ``q ``=` `Queue() ` `    ``q.put(``11``)  ` `    ``q.put(``12``)  ` `    ``q.put(``13``)  ` `    ``q.put(``14``)  ` `    ``q.put(``15``)  ` `    ``q.put(``16``)  ` `    ``q.put(``17``)  ` `    ``q.put(``18``)  ` `    ``q.put(``19``)  ` `    ``q.put(``20``)  ` `    ``interLeaveQueue(q)  ` `    ``length ``=` `q.qsize() ` `    ``for` `i ``in` `range``(length): ` `        ``print``(q.queue[``0``], end ``=` `" "``)  ` `        ``q.get() ` ` `  `# This code is contributed by PranchalK `

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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Improved By : PranchalKatiyar, Rajput-Ji

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