24. Swap Nodes in Pairs

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.

Note:

• Needs a dummy node to be the head of the list, return dummy.next
• Needs a prev to keep track of the node before the 1st node
• Needs a first node to keep track of the 1st node of the two nodes that needs swapped
• Needs a second node to keep track of the 2nd node of the two nodes that needs swapped
• Needs a remaining node to keep track of all the nodes after the second node

// C++ code
/**
 * 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) {
        if (head == NULL) {
            return NULL;
        }
        ListNode dummy = ListNode(0);
        ListNode* first = head; // 1st node of two adjacent nodes
        ListNode* second = head->next; // 2nd node of two adjacent nodes
        ListNode* prev = &dummy;
        while(second != NULL) {
            ListNode* remaining = second->next;
            second->next = first;
            prev->next = second;
            first->next = remaining;
            prev = first;
            first = remaining;
            if (first == NULL) {
                break;
            }
            second = first->next;
        }
        dummy.next = (dummy.next == NULL) ? first : dummy.next;
        return dummy.next;
    }
};