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;    }};