# Python Program For Moving All Occurrences Of An Element To End In A Linked List

Given a linked list and a key in it, the task is to move all occurrences of the given key to the end of the linked list, keeping the order of all other elements the same.

**Examples: **

Input : 1 -> 2 -> 2 -> 4 -> 3 key = 2 Output : 1 -> 4 -> 3 -> 2 -> 2 Input : 6 -> 6 -> 7 -> 6 -> 3 -> 10 key = 6 Output : 7 -> 3 -> 10 -> 6 -> 6 -> 6

A **simple solution** is to one by one find all occurrences of a given key in the linked list. For every found occurrence, insert it at the end. We do it till all occurrences of the given key are moved to the end.

**Time Complexity:** O(n^{2})

**Efficient Solution 1:** is to keep two pointers: **pCrawl** => Pointer to traverse the whole list one by one. **pKey** => Pointer to an occurrence of the key if a key is found. Else same as pCrawl.

We start both of the above pointers from the head of the linked list. We move **pKey** only when **pKey** is not pointing to a key. We always move **pCrawl**. So, when **pCrawl** and **pKey** are not the same, we must have found a key that lies before **pCrawl**, so we swap between **pCrawl** and **pKey**, and move **pKey** to the next location. The loop invariant is, after swapping of data, all elements from **pKey** to **pCrawl** are keys.

Below is the implementation of this approach.

## Python3

`# Python3 program to move all occurrences of a` `# given key to end.` ` ` `# Linked List node ` `class` `Node: ` ` ` `def` `__init__(` `self` `, data): ` ` ` `self` `.data ` `=` `data ` ` ` `self` `.` `next` `=` `None` ` ` `# A utility function to create a new node.` `def` `newNode(x):` ` ` ` ` `temp ` `=` `Node(` `0` `)` ` ` `temp.data ` `=` `x` ` ` `temp.` `next` `=` `None` ` ` `return` `temp` ` ` `# Utility function to print the elements` `# in Linked list` `def` `printList( head):` ` ` ` ` `temp ` `=` `head` ` ` `while` `(temp !` `=` `None` `) :` ` ` `print` `( temp.data,end ` `=` `" "` `)` ` ` `temp ` `=` `temp.` `next` ` ` ` ` `print` `()` ` ` `# Moves all occurrences of given key to` `# end of linked list.` `def` `moveToEnd(head, key):` ` ` ` ` `# Keeps track of locations where key` ` ` `# is present.` ` ` `pKey ` `=` `head` ` ` ` ` `# Traverse list` ` ` `pCrawl ` `=` `head` ` ` `while` `(pCrawl !` `=` `None` `) :` ` ` ` ` `# If current pointer is not same as pointer` ` ` `# to a key location, then we must have found` ` ` `# a key in linked list. We swap data of pCrawl` ` ` `# and pKey and move pKey to next position.` ` ` `if` `(pCrawl !` `=` `pKey ` `and` `pCrawl.data !` `=` `key) :` ` ` `pKey.data ` `=` `pCrawl.data` ` ` `pCrawl.data ` `=` `key` ` ` `pKey ` `=` `pKey.` `next` ` ` ` ` `# Find next position where key is present` ` ` `if` `(pKey.data !` `=` `key):` ` ` `pKey ` `=` `pKey.` `next` ` ` ` ` `# Moving to next Node` ` ` `pCrawl ` `=` `pCrawl.` `next` ` ` ` ` `return` `head` ` ` `# Driver code` `head ` `=` `newNode(` `10` `)` `head.` `next` `=` `newNode(` `20` `)` `head.` `next` `.` `next` `=` `newNode(` `10` `)` `head.` `next` `.` `next` `.` `next` `=` `newNode(` `30` `)` `head.` `next` `.` `next` `.` `next` `.` `next` `=` `newNode(` `40` `)` `head.` `next` `.` `next` `.` `next` `.` `next` `.` `next` `=` `newNode(` `10` `)` `head.` `next` `.` `next` `.` `next` `.` `next` `.` `next` `.` `next` `=` `newNode(` `60` `)` ` ` `print` `("Before moveToEnd(), the Linked ` `list` `is` `")` `printList(head)` ` ` `key ` `=` `10` `head ` `=` `moveToEnd(head, key)` ` ` `print` `("` `After moveToEnd(), the Linked ` `list` `is` `")` `printList(head)` ` ` `# This code is contributed by Arnab Kundu` |

**Output:**

Before moveToEnd(), the Linked list is 10 20 10 30 40 10 60 After moveToEnd(), the Linked list is 20 30 40 60 10 10 10

**Time Complexity:** O(n) requires only one traversal of the list.

**Efficient Solution 2 :**

1. Traverse the linked list and take a pointer at the tail.

2. Now, check for the key and node->data. If they are equal, move the node to last-next, else move ahead.

## Python3

`# Python3 code to remove key element to` `# end of linked list` ` ` `# A Linked list Node` `class` `Node:` ` ` ` ` `def` `__init__(` `self` `, data):` ` ` ` ` `self` `.data ` `=` `data` ` ` `self` `.` `next` `=` `None` ` ` `# A utility function to create a new node.` `def` `newNode(x):` ` ` ` ` `temp ` `=` `Node(x)` ` ` `return` `temp` ` ` `# Function to remove key to end` `def` `keyToEnd(head, key):` ` ` ` ` `# Node to keep pointing to tail` ` ` `tail ` `=` `head` ` ` ` ` `if` `(head ` `=` `=` `None` `):` ` ` `return` `None` ` ` ` ` `while` `(tail.` `next` `!` `=` `None` `): ` ` ` `tail ` `=` `tail.` `next` ` ` ` ` `# Node to point to last of linked list` ` ` `last ` `=` `tail` ` ` `current ` `=` `head` ` ` `prev ` `=` `None` ` ` ` ` `# Node prev2 to point to previous` ` ` `# when head.data!=key` ` ` `prev2 ` `=` `None` ` ` ` ` `# Loop to perform operations to ` ` ` `# remove key to end` ` ` `while` `(current !` `=` `tail):` ` ` `if` `(current.data ` `=` `=` `key ` `and` `prev2 ` `=` `=` `None` `):` ` ` `prev ` `=` `current` ` ` `current ` `=` `current.` `next` ` ` `head ` `=` `current` ` ` `last.` `next` `=` `prev` ` ` `last ` `=` `last.` `next` ` ` `last.` `next` `=` `None` ` ` `prev ` `=` `None` ` ` ` ` `else` `:` ` ` `if` `(current.data ` `=` `=` `key ` `and` `prev2 !` `=` `None` `):` ` ` `prev ` `=` `current` ` ` `current ` `=` `current.` `next` ` ` `prev2.` `next` `=` `current` ` ` `last.` `next` `=` `prev` ` ` `last ` `=` `last.` `next` ` ` `last.` `next` `=` `None` ` ` ` ` `elif` `(current !` `=` `tail):` ` ` `prev2 ` `=` `current` ` ` `current ` `=` `current.` `next` ` ` ` ` `return` `head` ` ` `# Function to display linked list` `def` `printList(head):` ` ` ` ` `temp ` `=` `head` ` ` ` ` `while` `(temp !` `=` `None` `):` ` ` `print` `(temp.data, end ` `=` `' '` `)` ` ` `temp ` `=` `temp.` `next` ` ` ` ` `print` `()` ` ` `# Driver Code` `if` `__name__` `=` `=` `'__main__'` `:` ` ` ` ` `root ` `=` `newNode(` `5` `)` ` ` `root.` `next` `=` `newNode(` `2` `)` ` ` `root.` `next` `.` `next` `=` `newNode(` `2` `)` ` ` `root.` `next` `.` `next` `.` `next` `=` `newNode(` `7` `)` ` ` `root.` `next` `.` `next` `.` `next` `.` `next` `=` `newNode(` `2` `)` ` ` `root.` `next` `.` `next` `.` `next` `.` `next` `.` `next` `=` `newNode(` `2` `)` ` ` `root.` `next` `.` `next` `.` `next` `.` `next` `.` `next` `.` `next` `=` `newNode(` `2` `)` ` ` ` ` `key ` `=` `2` ` ` `print` `(` `"Linked List before operations :"` `)` ` ` `printList(root)` ` ` `print` `(` `"Linked List after operations :"` `)` ` ` `root ` `=` `keyToEnd(root, key)` ` ` ` ` `printList(root)` ` ` `# This code is contributed by rutvik_56` |

**Output:**

Linked List before operations : 5 2 2 7 2 2 2 Linked List after operations : 5 7 2 2 2 2 2

Thanks to **Ravinder Kumar** for suggesting this method.

**Efficient Solution 3:** is to maintain a separate list of keys. We initialize this list of keys as empty. We traverse the given list. For every key found, we remove it from the original list and insert it into a separate list of keys. We finally link the list of keys at the end of the remaining given list. The time complexity of this solution is also O(n) and it also requires only one traversal of the list.

Please refer complete article on Move all occurrences of an element to end in a linked list for more details!