Implement swapPairs function to swap nodes

[aryaman0406]

PR Title Format: 24.Swap Nodes in Pairs.cpp

💡 Intuition
The core idea is to perform a group-wise reversal of linked list nodes, where each group has a size of two. Since the node pointers must be correctly updated to maintain the list structure, an iterative approach using a dummy node simplifies the logic, especially the handling of the new head and linking the swapped pair back to the previous segment of the list.

✍️ Approach
This solution uses an iterative approach with a dummy head node and three pointers to manage the swapping: prev, cur (the first node of the pair), and second (the second node of the pair).

Dummy Node Initialization: A dummy node is created and points to the original head. This node acts as a fixed starting point and simplifies the logic for updating the pointer that precedes the swapped pair. The pointer prev is initialized to point to the dummy node.

Iteration Setup: The pointer cur is initialized to the head of the list. The loop continues as long as a valid pair exists: while (cur && cur->next).

Swapping Steps (within the loop):

Identify: Identify the critical nodes:

second: The second node of the pair (cur->next).

npn (Next Pair Node): The first node of the next pair (cur->next->next). This is stored so we don't lose the rest of the list.

Reverse Pointers:

Link the second node to the first: second->next = cur; (Swap)

Link the original first node to the next unswapped segment: cur->next = npn; (Maintain continuity)

Link the preceding node to the new first node (which is second): prev->next = second; (Attach the swapped pair)

Advance Pointers:

prev moves to the unswapped node (cur), which is now the end of the swapped pair.

cur moves to the start of the next pair (npn).

Return: After the loop completes, the new head of the list is dummy.next.

Code Solution (C++)
C++

/**

• Definition for singly-linked list.

• struct ListNode {

• int val;

• ListNode *next;

• ListNode() : val(0), next(nullptr) {}

• ListNode(int x) : val(x), next(nullptr) {}

• ListNode(int x, ListNode *next) : val(x), next(next) {}

• };
  /
  class Solution {
  public:
  ListNode swapPairs(ListNode* head) {
  // Create a dummy node to act as the predecessor to the head
  ListNode dummy(0, head);
  ListNode *prev = &dummy;
  ListNode *cur = head;

   // Loop while there is at least one pair remaining (cur and cur->next are non-null)
   while (cur && cur->next) {
       // 1. Identify critical nodes for swapping
       ListNode *second = cur->next; 
       ListNode *npn = cur->next->next; // Start of the next pair
  
       // 2. Perform the swap
       second->next = cur;     // Point second node to first node (cur)
       cur->next = npn;        // Point first node to the next pair's start (npn)
       prev->next = second;    // Link the previous segment to the new pair's start (second)
  
       // 3. Advance pointers for the next iteration
       prev = cur;             // The original first node (cur) is now the predecessor for the next swap
       cur = npn;              // Move to the start of the next pair
   }
  
   return dummy.next;        
  

  }
  };
  🔗 Related Issues
  By submitting this PR, I confirm that:

[x] This is my original work not totally AI generated

[x] I have tested the solution thoroughly on leetcode

[x] I have maintained proper PR description format

[x] This is a meaningful contribution, not spam

Summary by Sourcery

New Features:

• Add swapPairs function to swap adjacent nodes in a linked list in C++ using a dummy head and pointer manipulation.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Introduced a new C++ implementation of swapPairs using an iterative dummy-head approach that performs in-place pairwise node swaps in a singly-linked list.

File-Level Changes

Change	Details	Files
Implement swapPairs using an iterative three-pointer technique	Initialize dummy head and pointers (prev, cur) Add loop to process nodes while cur and cur->next exist Identify pair nodes (second, npn) and update next pointers to swap Advance prev to cur and cur to npn for next iteration Return new head via dummy.next	24.Swap Nodes in Pairs.cpp

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