Leetcode: 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.

比较简单，指针操作。

/**
 * 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 || head->next == NULL) {
            return head;
        }
        
        ListNode *first = head;
        ListNode *second = head->next;
        ListNode *newhead = second;
        ListNode *tmp = NULL;
        ListNode *prev = NULL;
        while (first != NULL && second != NULL) {
            tmp = second->next;
            second->next = first;
            first->next = tmp;
            if (prev != NULL) {
                prev->next = second;
            }
            
            prev = first;
            first = tmp;
            if (tmp != NULL) {
                second = tmp->next;
            }
        }
        
        return newhead;
    }
};