Swap Pairs in linklist

Problem:

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

Solution:


/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *swapPairs(ListNode *head) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (head == NULL )
            return NULL;
        if (head->next == NULL)
            return head;
        ListNode *temp = NULL, *temp2 = NULL, *temp3 = NULL, *newHead = NULL, *prev = NULL;
        temp = head;
        newHead = head->next;
     
        while (temp != NULL) {
            temp3 = temp;
            temp2 = temp->next;
            if (temp2 != NULL) {
                temp->next = temp2->next;
                temp2->next = temp3;
                if (prev != NULL)
                    prev->next = temp2;
            }
            prev = temp3;
            temp = temp->next;
        }
        return newHead;
    }
};

=========================
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        // Recursive. ============================
        /*
        if (head == NULL || head -> next == NULL) {
            return head;
        } else {
            ListNode *tmp = head -> next;
            head -> next = swapPairs(head -> next -> next);
            tmp -> next = head;
            return tmp;
        }
        */
        // Iterative. ===============================
        if (head == NULL || head -> next == NULL) {
            return head;
        }
        ListNode *curr = head;
        ListNode *ans = NULL, *prev = NULL, *next = NULL;
        while (curr != NULL && curr -> next != NULL) {
            if (prev == NULL) {
                ans = curr -> next;
            } else {
                prev -> next = curr -> next;
            }
            prev = curr;
            next = curr -> next -> next;
            curr -> next -> next = curr;
            curr = next;
        }
        if (curr != NULL && curr -> next == NULL) {
            prev -> next = curr;
        } else {
            prev -> next = NULL;
        }
        return ans;
    }
};