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.

 

设置个头结点，便于操作，每两个就反转

/**
 * 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 ) return head;
        ListNode node(-1);  //设置头结点
        node.next = head;
        ListNode* pre = &node;
        ListNode* p = head;
        while( p && p->next ) { //如果当前节点及下一个节点存在
            pre->next = p->next;
            p->next = p->next->next;
            pre->next->next = p;
            pre = p;
            p = p->next;
        }
        return node.next;
    }
};