main.cpp

/// Source : https://leetcode.com/problems/swap-nodes-in-pairs/description/
/// Author : liuyubobobo
/// Time   : 2017-11-15

#include <iostream>

using namespace std;

/// Definition for singly-linked list.
struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

/// LinkedList Test Helper Functions
ListNode* createLinkedList(int arr[], int n){

    if(n == 0)
        return NULL;

    ListNode* head = new ListNode(arr[0]);
    ListNode* curNode = head;
    for(int i = 1 ; i < n ; i ++){
        curNode->next = new ListNode(arr[i]);
        curNode = curNode->next;
    }

    return head;
}

void printLinkedList(ListNode* head){

    if(head == NULL){
        cout<<"NULL"<<endl;
        return;
    }

    ListNode* curNode = head;
    while(curNode != NULL){
        cout << curNode->val;
        if(curNode->next != NULL)
            cout << " -> ";
        curNode = curNode->next;
    }

    cout << endl;

    return;
}

void deleteLinkedList(ListNode* head){

    ListNode* curNode = head;
    while(curNode != NULL){
        ListNode* delNode = curNode;
        curNode = curNode->next;
        delete delNode;
    }

    return;
}

// Time Complexity: O(n)
// Space Complexity: O(1)
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {

        ListNode* dummyHead = new ListNode(0);
        dummyHead->next = head;

        ListNode* p = dummyHead;
        while(p->next && p->next->next){
            ListNode* node1 = p->next;
            ListNode* node2 = node1->next;
            ListNode* next = node2->next;
            node2->next = node1;
            node1->next = next;
            p->next = node2;
            p = node1;
        }

        ListNode* retHead = dummyHead->next;
        delete dummyHead;

        return retHead;
    }
};

int main() {

    int arr[] = {1, 2, 3, 4};
    int n = sizeof(arr) / sizeof(int);

    ListNode* head = createLinkedList(arr, n);
    printLinkedList(head);

    head = Solution().swapPairs(head);
    printLinkedList(head);

    deleteLinkedList(head);

    return 0;
}