# Swap first odd and even valued nodes from the beginning and end of a Linked List

Given a singly Linked List, the task is to swap the first odd valued node from the beginning and the first even valued node from the end of the Linked List. If the list contains node values of a single parity, then no modifications are required.

**Examples:**

Input:4 -> 3 -> 5 -> 2 -> 3 -> NULLOutput:4 -> 2 -> 5 -> 3 -> 3 -> NULLExplanation:

4 ->3-> 5 ->2-> 3 -> NULL ===> 4 ->2-> 5 ->3-> 3 -> NULL

The first odd value in any node from the beginning is3.

The first even value in any node from the end is2.

After swapping the above two node values, the linked list modifies to 4 -> 2 -> 5 -> 3 -> 3 -> NULL.

Input:LL: 2 -> 6 -> 8 -> 2 -> NULLOutput:2 -> 6 -> 8 -> 2 -> NULL

**Approach:** The given problem can be solved by keeping track of the first and the last occurrences of odd and even valued nodes respectively and swapping them. Follow the steps below to solve the problem:

- Initialize two variables, say
**firstOdd**and**firstEven**, to store the first node having odd and even values from the beginning and the end respectively. - Initialize two variables, say
**firstOdd**and**firstEven**(initially NULL). - Traverse the linked list and perform the following steps:
- If the value of the current node is
**odd**and**firstOdd**is**NULL**, then update the**firstOdd**node to the current node. - Otherwise, update the
**firstEven**as the current node.

- If the value of the current node is
- After completing the above steps, if
**firstOdd**and**firstEven**is not**NULL**, then swap the values at both the pointers. - Print the modified Linked List as the resultant Linked List.

Below is the implementation of the above approach:

## Python3

`# Python3 program for the above approach ` ` ` `# Structure of a node ` `# in the Linked List ` `class` `Node: ` ` ` `def` `__init__(` `self` `, x): ` ` ` `self` `.val ` `=` `x ` ` ` `self` `.` `next` `=` `None` ` ` `# Function to display the Linked List ` `def` `printLL(head): ` ` ` ` ` `# Traverse until end ` ` ` `# of list is reached ` ` ` `while` `(head): ` ` ` ` ` `# Print the value ` ` ` `# stored in the head ` ` ` `print` `(head.val, end ` `=` `' '` `) ` ` ` ` ` `# Move to the next of head ` ` ` `head ` `=` `head.` `next` ` ` `print` `() ` ` ` `# Function to swap the nodes ` `def` `swapNodes(head, even, odd): ` ` ` ` ` `# Keeps the track of ` ` ` `# prevEven and CurrEven ` ` ` `prevEven ` `=` `None` ` ` `currEven ` `=` `head ` ` ` ` ` `while` `currEven ` `and` `currEven !` `=` `even: ` ` ` `prevEven ` `=` `currEven ` ` ` `currEven ` `=` `currEven.` `next` ` ` ` ` `# Keeps the track of ` ` ` `# prevOdd and currOdd ` ` ` `prevOdd ` `=` `None` ` ` `currOdd ` `=` `head ` ` ` ` ` `while` `currOdd ` `and` `currOdd !` `=` `odd: ` ` ` `prevOdd ` `=` `currOdd ` ` ` `currOdd ` `=` `currOdd.` `next` ` ` ` ` `# If list contains nodes ` ` ` `# of a single parity ` ` ` `if` `not` `currEven ` `or` `not` `currOdd: ` ` ` `return` `head ` ` ` ` ` `# If head of the linked list ` ` ` `# does not contain even value ` ` ` `if` `prevEven: ` ` ` `prevEven.` `next` `=` `currOdd ` ` ` ` ` `# Make odd node the new head ` ` ` `else` `: ` ` ` `head ` `=` `currOdd ` ` ` ` ` `# If head of the linked list ` ` ` `# does not contain odd value ` ` ` `if` `prevOdd: ` ` ` `prevOdd.` `next` `=` `currEven ` ` ` ` ` `# Make even node the new head ` ` ` `else` `: ` ` ` `head ` `=` `currEven ` ` ` ` ` `# Swap the next pointers ` ` ` `temp ` `=` `currEven.` `next` ` ` `currEven.` `next` `=` `currOdd.` `next` ` ` `currOdd.` `next` `=` `temp ` ` ` ` ` `# Return the modified Linked List ` ` ` `return` `head ` ` ` `# Function to swap the first odd node ` `# from the beginning and the first even ` `# node from the end of the Linked List ` `def` `swapOddAndEvenNodes(head): ` ` ` ` ` `# Find the first even node from ` ` ` `# the end of the Linked List ` ` ` `even ` `=` `None` ` ` `curr ` `=` `head ` ` ` `while` `curr: ` ` ` `if` `not` `curr.val & ` `1` `: ` ` ` `even ` `=` `curr ` ` ` `curr ` `=` `curr.` `next` ` ` ` ` `# Find the first odd node from ` ` ` `# the front of the Linked List ` ` ` `odd ` `=` `None` ` ` `curr ` `=` `head ` ` ` `while` `curr: ` ` ` `if` `curr.val & ` `1` `: ` ` ` `odd ` `=` `curr ` ` ` `break` ` ` `curr ` `=` `curr.` `next` ` ` ` ` `# If required odd and even ` ` ` `# nodes are found, then swap ` ` ` `if` `odd ` `and` `even: ` ` ` `head ` `=` `swapNodes(head, even, odd) ` ` ` ` ` `printLL(head) ` ` ` `# Function to convert given ` `# array into a Linked List ` `def` `linkedList(arr): ` ` ` `head ` `=` `None` ` ` `ptr ` `=` `None` ` ` ` ` `# 4 -> 3 -> 5 -> 2 -> 3 -> NULL ` ` ` `for` `i ` `in` `arr: ` ` ` `if` `not` `head: ` ` ` `head ` `=` `Node(i) ` ` ` `ptr ` `=` `head ` ` ` `else` `: ` ` ` `newNode ` `=` `Node(i) ` ` ` `ptr.` `next` `=` `newNode ` ` ` `ptr ` `=` `newNode ` ` ` `return` `head ` ` ` `# Driver Code ` ` ` `# Given Linked List ` `arr ` `=` `[` `4` `, ` `3` `, ` `5` `, ` `2` `, ` `3` `] ` ` ` `# Stores head of Linked List ` `head ` `=` `linkedList(arr) ` ` ` `swapOddAndEvenNodes(head) ` |

**Output:**

4 2 5 3 3

**Time Complexity:** O(N)**Auxiliary Space:** O(1)