Swap Nodes in Paris

[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++)

/**
 * 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:

• This is my original work not totally AI generated

• I have tested the solution thoroughly on leetcode

• I have maintained proper PR description format

• This is a meaningful contribution, not spam

Summary by Sourcery

New Features:

• Implement swapPairs method using an iterative dummy-node approach to reverse nodes in groups of two.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Introduces an iterative solution for swapping nodes in pairs within a singly linked list by using a dummy head and three pointers (prev, cur, second) to manage pointer updates and maintain list continuity.

File-Level Changes

Change	Details	Files
Initialize dummy head and pointers	Create a dummy node pointing to the original head Set prev to the dummy node and cur to head	24.Swap Nodes in Pairs.cpp
Implement pair-wise swapping loop	Loop while cur and cur->next are non-null Identify second (cur->next) and npn (cur->next->next) nodes Reverse pointers: second->next = cur, cur->next = npn Link prev->next to second	24.Swap Nodes in Pairs.cpp
Advance pointers after each swap	Move prev to the original first node (cur) Set cur to the start of the next pair (npn)	24.Swap Nodes in Pairs.cpp
Return new head	Return dummy.next as the head of the modified list	24.Swap Nodes in Pairs.cpp

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[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• The PR title says “Swap Nodes in Paris” instead of “Pairs”—please fix the typo.
• The diff file path shows “197. Rising Temperature” which doesn’t match this swapPairs solution—update the filename to reflect the actual implementation.
• Consider renaming the npn variable to something more descriptive (e.g. nextPair) for improved readability.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The PR title says “Swap Nodes in Paris” instead of “Pairs”—please fix the typo.
- The diff file path shows “197. Rising Temperature” which doesn’t match this swapPairs solution—update the filename to reflect the actual implementation.
- Consider renaming the `npn` variable to something more descriptive (e.g. `nextPair`) for improved readability.

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[SjxSubham]

@aryaman0406 do these changes

1. U are only allowed to add upto one single problem in a single PR...
   so delete those xtra files and add only one file that justifies ur PR description
2. raise an ISSUE as well
3. rename PR title with PRoper format ProblemNo. ProblemName.cpp
4. Star the repo ⭐