1721. Swapping Nodes in a Linked List

Description

You are given the head of a linked list, and an integer k.

Return the head of the linked list after swapping the values of the kth node from the beginning and the kth node from the end (the list is 1-indexed).

Constraints

• The number of nodes in the list is n.

• 1 <= k <= n <= 105

• 0 <= Node.val <= 100

Approach

Links

• GeeksforGeeks

• Leetcode

• ProgramCreek

• YouTube

Examples

Example 1

Input: head = [1, 2, 3, 4, 5], k = 2

Output: [1, 4, 3, 2, 5]

Example 2

Input: head = [7, 9, 6, 6, 7, 8, 3, 0, 9, 5], k = 5

Output: [7, 9, 6, 6, 8, 7, 3, 0, 9, 5]

Example 3

Input: head = [1], k = 1

Output: [1]

Example 4

Input: head = [1, 2], k = 1

Output: [2, 1]

Example 5

Input: head = [1, 2, 3], k = 2

Output: [1, 2, 3]

Solutions

ListNode

// Definition for singly-linked list.
public class ListNode {
	int val;
	ListNode next;
	ListNode() {}
	ListNode(int val) {
		this.val = val;
	}
	ListNode(int val, ListNode next) { 
		this.val = val; this.next = next;
	}
}

Solution 1

/**
 * Time complexity : O(N)
 * Space complexity : O(1)
 */

class Solution {
    public ListNode swapNodes(ListNode head, int k) {
        if(head == null || (head.next == null && k == 1)) {
            return head;
        }
        
        ListNode kthNodeFromStart = head, kthNodeFromEnd = head, temp = head;
        
        while(k-- > 1) {
            kthNodeFromStart = kthNodeFromStart.next;
            temp = temp.next;
        }
        
        while(temp.next != null) {
            kthNodeFromEnd = kthNodeFromEnd.next;
            temp = temp.next;
        }
        
        int val = kthNodeFromStart.val;
        kthNodeFromStart.val = kthNodeFromEnd.val;
        kthNodeFromEnd.val = val;
        
        return head;
    }
}

Solution 2

/**
 * Time complexity : O(n), where n is the size of Linked List. 
 *    We are iterating over the entire Linked List once.
 * Space complexity : O(1), as we are using constant extra space to maintain 
 *    list node pointers frontNode, endNode and currentNode.
 */
 
 class Solution {
    public ListNode swapNodes(ListNode head, int k) {
        int listLength = 0;
        ListNode frontNode = null;
        ListNode endNode = null;
        ListNode currentNode = head;
        
        // set the front node and end node in single pass
        while (currentNode != null) {
            listLength++;
            if (endNode != null)
                endNode = endNode.next;
            // check if we have reached kth node
            if (listLength == k) {
                frontNode = currentNode;
                endNode = head;
            }
            currentNode = currentNode.next;
        }
        
        // swap the values of front node and end node
        int temp = frontNode.val;
        frontNode.val = endNode.val;
        endNode.val = temp;
        
        return head;
    }
}

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