[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.

利用三个指针p, pPre, pPrePre来操作，画个图就比较清晰了

/**
 * 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;
            
        ListNode *pPrePre = NULL;
        ListNode *pPre = NULL;
        ListNode *p = head;
        
        while(p && p->next)
        {
            pPre = p;
            p = p->next;
            
            ListNode *pNext = p->next;
            
            if (pPre == head)
                head = p;
                
            if (pPrePre)
                pPrePre->next = p;
                
            p->next = pPre;
            pPre->next = pNext;
            
            pPrePre = pPre;
            p = pNext;
        }
        
        return head;
    }
};