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【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.

一A的水题，哦啦啦啦啦~

就注意一点：不能光顾着该当前的两个链表节点的指针，还要用last_pair存放上一组最后一个节点的指针，以修改这个节点的next指针指向当前组新的第一个指针。如果没有上一组时候要特殊处理head指针。有点饶，举个例子，题目给的例子里面，当交换1，2的时候注意head指针要指向2而不是1；交换完1，2得到如下链表2->1->3->4，当交换3，4的时候注意要修改1的next指针指向4。

代码如下：

 1 #include <iostream>
 2 using namespace std;
 3 struct ListNode {
 4     int val;
 5     ListNode *next;
 6      ListNode(int x) : val(x), next(NULL) {}
 7  };
 8 
 9 class Solution {
10 public:
11     ListNode *swapPairs(ListNode *head) {
12         ListNode* current = head;
13         ListNode* last_pair = head;
14         while(current != NULL){
15             ListNode* temp = current->next;
16             if(temp != NULL){
17                 if(last_pair == head)
18                     head = temp;
19                 else
20                     last_pair->next = temp;
21                 last_pair = current;    
22                 current->next = temp->next;
23                 temp->next = current;
24             }
25 
26             current = current->next;
27         }
28         return head;
29     }
30 };
31 
32 int main(){
33     Solution s;
34     ListNode* head = new ListNode(1);
35     ListNode* node1= new ListNode(2);
36     head->next = node1;
37 
38     ListNode* node2= new ListNode(3);
39     node1->next = node2;
40 
41     ListNode* node3= new ListNode(4);
42     node2->next = node3;
43 
44     ListNode* r = s.swapPairs(head);
45     while(r != NULL){
46         cout <<r->val<<endl;
47         r = r->next;
48     }
49 }

JAVA版本：

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode swapPairs(ListNode head) {
14         if(head == null)
15             return head;
16         
17         ListNode newHead = null;
18         ListNode one = head;
19         ListNode two = head.next;
20         ListNode before = null;
21         
22         while(one != null && two != null){
23             ListNode three = two.next;
24             two.next = one;
25             one.next = three;
26             
27             if(newHead == null)
28             {
29                 newHead = two;
30                 before = one;
31             }else {
32                 before.next = two;
33                 before = one;
34             }  
35             one = three;
36             if(one != null)
37                 two = one.next;
38         }
39         if(one != null && before != null)
40             before.next = one;
41         return newHead==null?head:newHead;
42     }
43 }