Swap Nodes in Pairs Leetcode Solution

In this post, we are going to solve the Swap Nodes in Pairs Leetcode Solution problem of Leetcode. This Leetcode problem is done in many programming languages like C++, Java, and Python.

Table of Contents

Problem

Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the list’s nodes (i.e., only nodes themselves may be changed.)

Example 1:

Input: head = [1,2,3,4]
Output: [2,1,4,3]

Example 2:

Input: head = []
Output: []

Example 3:

Input: head = [1]
Output: [1]

Constraints:

The number of nodes in the list is in the range [0, 100].
0 <= Node.val <= 100

Now, let’s see the leetcode solution of Swap Nodes in Pairs Leetcode Solution.

Swap Nodes in Pairs Leetcode Solution in Python

class Solution:
  def swapPairs(self, head: ListNode) -> ListNode:
    def getLength(head: ListNode) -> int:
      length = 0
      while head:
        length += 1
        head = head.next
      return length

    length = getLength(head)
    dummy = ListNode(0, head)
    prev = dummy
    curr = head

    for _ in range(length // 2):
      next = curr.next
      curr.next = next.next
      next.next = prev.next
      prev.next = next
      prev = curr
      curr = curr.next

    return dummy.next

Swap Nodes in Pairs Leetcode Solution in CPP

class Solution {
 public:
  ListNode* swapPairs(ListNode* head) {
    const int length = getLength(head);
    ListNode dummy(0, head);
    ListNode* prev = &dummy;
    ListNode* curr = head;

    for (int i = 0; i < length / 2; ++i) {
      ListNode* next = curr->next;
      curr->next = next->next;
      next->next = prev->next;
      prev->next = next;
      prev = curr;
      curr = curr->next;
    }

    return dummy.next;
  }

 private:
  int getLength(ListNode* head) {
    int length = 0;
    for (ListNode* curr = head; curr; curr = curr->next)
      ++length;
    return length;
  }
};

Swap Nodes in Pairs Leetcode Solution in Java

class Solution {
  public ListNode swapPairs(ListNode head) {
    final int length = getLength(head);
    ListNode dummy = new ListNode(0, head);
    ListNode prev = dummy;
    ListNode curr = head;

    for (int i = 0; i < length / 2; ++i) {
      ListNode next = curr.next;
      curr.next = next.next;
      next.next = curr;
      prev.next = next;
      prev = curr;
      curr = curr.next;
    }

    return dummy.next;
  }

  private int getLength(ListNode head) {
    int length = 0;
    for (ListNode curr = head; curr != null; curr = curr.next)
      ++length;
    return length;
  }
}

Note: This problem Swap Nodes in Pairs is generated by Leetcode but the solution is provided by Chase2learn This tutorial is only for Educational and Learning purposes.

NOTE: Reverse Nodes in k-Group Leetcode Solution