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;
    }
};