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.

我的答案：

方案一：交换结点

/**
* 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 *tempHead=head->next,*priHead=head;
while(head!=NULL&&head->next!=NULL)
{
ListNode *temp=head->next;
priHead->next=temp;
head->next=temp->next;
temp->next=head;
priHead=head;
head=head->next;
}
return tempHead;
}
};

方案二：交换结点中的值

/**
* 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 *tempHead=head;
while(head!=NULL&&head->next!=NULL){
int tempVal=head->val;
head->val=head->next->val;
head->next->val=tempVal;
head=head->next->next;
}
return tempHead;
}
};