Given a Linked List of integers. The task is to write a program to modify the linked list such that all even numbers appear before all the odd numbers in the modified linked list. It is not needed to keep the order of even and odd nodes same as that of the original list, the task is just to rearrange the nodes such that all even valued nodes appear before the odd valued nodes.

**See Also**: Segregate even and odd nodes in a Linked List

**Examples**:

Input: 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 -> 10 -> NULL

Output: 10 -> 8 -> 6 -> 4 -> 2 -> 1 -> 3 -> 5 -> 7 -> 9 -> NULL

Input: 4 -> 3 -> 2 -> 1 -> NULL

Output: 2 -> 4 -> 3 -> 1 -> NULL

The idea is to iteratively push all the elements of the linked list to deque as per the below conditions:

- Start traversing the linked list and if an element is even then push it to the front of the Deque and,
- If the element is odd then push it to the back of the Deque.

Finally, replace all elements of the linked list with the elements of Deque starting from the first element.

Below is the implementation of the above approach:

## C++

`// CPP program to segregate even and ` `// odd noeds in a linked list using deque ` `#include <bits/stdc++.h> ` `using` `namespace` `std; ` ` ` `/* Link list node */` `struct` `Node { ` ` ` `int` `data; ` ` ` `struct` `Node* next; ` `}; ` ` ` `/*UTILITY FUNCTIONS*/` `/* Push a node to linked list. Note that this function ` `changes the head */` `void` `push(` `struct` `Node** head_ref, ` `char` `new_data) ` `{ ` ` ` `/* allocate node */` ` ` `struct` `Node* new_node = (` `struct` `Node*)` `malloc` `(` `sizeof` `(` `struct` `Node)); ` ` ` ` ` `/* put in the data */` ` ` `new_node->data = new_data; ` ` ` ` ` `/* link the old list off the new node */` ` ` `new_node->next = (*head_ref); ` ` ` ` ` `/* move the head to pochar to the new node */` ` ` `(*head_ref) = new_node; ` `} ` ` ` `// printing the linked list ` `void` `printList(` `struct` `Node* head) ` `{ ` ` ` `struct` `Node* temp = head; ` ` ` `while` `(temp != NULL) { ` ` ` `printf` `(` `"%d "` `, temp->data); ` ` ` `temp = temp->next; ` ` ` `} ` `} ` ` ` `// Function to rearrange even and odd ` `// elements in a linked list using deque ` `void` `evenOdd(` `struct` `Node* head) ` `{ ` ` ` `struct` `Node* temp = head; ` ` ` ` ` `// Declaring a Deque ` ` ` `deque<` `int` `> d; ` ` ` ` ` `// Push all the elements of ` ` ` `// linked list in to deque ` ` ` `while` `(temp != NULL) { ` ` ` ` ` `// if element is even push it ` ` ` `// to front of the deque ` ` ` `if` `(temp->data % 2 == 0) ` ` ` `d.push_front(temp->data); ` ` ` ` ` `else` `// else push at the back of the deque ` ` ` `d.push_back(temp->data); ` ` ` `temp = temp->next; ` `// increase temp ` ` ` `} ` ` ` ` ` `temp = head; ` ` ` ` ` `// Replace all elements of the linked list ` ` ` `// with the elements of Deque starting from ` ` ` `// the first element ` ` ` `while` `(!d.empty()) { ` ` ` `temp->data = d.front(); ` ` ` `d.pop_front(); ` ` ` `temp = temp->next; ` ` ` `} ` `} ` ` ` `// Driver code ` `int` `main() ` `{ ` ` ` `struct` `Node* head = NULL; ` ` ` `push(&head, 10); ` ` ` `push(&head, 9); ` ` ` `push(&head, 8); ` ` ` `push(&head, 7); ` ` ` `push(&head, 6); ` ` ` `push(&head, 5); ` ` ` `push(&head, 4); ` ` ` `push(&head, 3); ` ` ` `push(&head, 2); ` ` ` `push(&head, 1); ` ` ` ` ` `cout << ` `"Given linked list: "` `; ` ` ` `printList(head); ` ` ` ` ` `evenOdd(head); ` ` ` ` ` `cout << ` `"\nAfter rearrangement: "` `; ` ` ` `printList(head); ` ` ` ` ` `return` `0; ` `} ` |

*chevron_right*

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## Python

`# Python program to segregate even and ` `# odd noeds in a linked list using deque ` `import` `collections ` ` ` `# Node class ` `class` `Node: ` ` ` ` ` `# Function to initialise the node object ` ` ` `def` `__init__(` `self` `, data): ` ` ` `self` `.data ` `=` `data ` `# Assign data ` ` ` `self` `.` `next` `=` `None` ` ` `# UTILITY FUNCTIONS ` `# Push a node to linked list. Note that this function ` `# changes the head ` `def` `push( head_ref, new_data): ` ` ` ` ` `# allocate node ` ` ` `new_node ` `=` `Node(` `0` `) ` ` ` ` ` `# put in the data ` ` ` `new_node.data ` `=` `new_data ` ` ` ` ` `# link the old list off the new node ` ` ` `new_node.` `next` `=` `(head_ref) ` ` ` ` ` `# move the head to pochar to the new node ` ` ` `(head_ref) ` `=` `new_node ` ` ` ` ` `return` `head_ref ` ` ` `# printing the linked list ` `def` `printList( head): ` ` ` ` ` `temp ` `=` `head ` ` ` `while` `(temp !` `=` `None` `): ` ` ` `print` `( temp.data, end ` `=` `" "` `) ` ` ` `temp ` `=` `temp.` `next` ` ` `# Function to rearrange even and odd ` `# elements in a linked list using deque ` `def` `evenOdd( head): ` ` ` ` ` `temp ` `=` `head ` ` ` ` ` `# Declaring a Deque ` ` ` `d ` `=` `collections.deque([]) ` ` ` ` ` `# Push all the elements of ` ` ` `# linked list in to deque ` ` ` `while` `(temp !` `=` `None` `) : ` ` ` ` ` `# if element is even push it ` ` ` `# to front of the deque ` ` ` `if` `(temp.data ` `%` `2` `=` `=` `0` `): ` ` ` `d.appendleft(temp.data) ` ` ` ` ` `else` `: ` `# else push at the back of the deque ` ` ` `d.append(temp.data) ` ` ` `temp ` `=` `temp.` `next` `# increase temp ` ` ` ` ` `temp ` `=` `head ` ` ` ` ` `# Replace all elements of the linked list ` ` ` `# with the elements of Deque starting from ` ` ` `# the first element ` ` ` `while` `(` `len` `(d) > ` `0` `) : ` ` ` `temp.data ` `=` `d[` `0` `] ` ` ` `d.popleft() ` ` ` `temp ` `=` `temp.` `next` ` ` `# Driver code ` ` ` `head ` `=` `None` `head ` `=` `push(head, ` `10` `) ` `head ` `=` `push(head, ` `9` `) ` `head ` `=` `push(head, ` `8` `) ` `head ` `=` `push(head, ` `7` `) ` `head ` `=` `push(head, ` `6` `) ` `head ` `=` `push(head, ` `5` `) ` `head ` `=` `push(head, ` `4` `) ` `head ` `=` `push(head, ` `3` `) ` `head ` `=` `push(head, ` `2` `) ` `head ` `=` `push(head, ` `1` `) ` ` ` `print` `( ` `"Given linked list: "` `, end ` `=` `"") ` `printList(head) ` ` ` `evenOdd(head) ` ` ` `print` `(` `"\nAfter rearrangement: "` `, end ` `=` `"") ` `printList(head) ` ` ` `# This code is contributed by Arnab Kundu ` |

*chevron_right*

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**Output:**

Given linked list: 1 2 3 4 5 6 7 8 9 10 After rearrangement: 10 8 6 4 2 1 3 5 7 9

**Time complexity**: O(N)

**Auxiliary Space**: O(N), where N is the total number of nodes in the linked list.

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