7两两交换链表中的节点（24）

4
1 2 3 4

head-->2-->1-->4-->3-->tail

#include<iostream>
#include<vector>
using namespace std;

struct ListNode
{
    int val;
    ListNode *next;
    ListNode() : val(0), next(NULL) {}
    ListNode(int x) : val(x), next(NULL) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};
class Solution {
public:
    ListNode* swapPairs(ListNode* head) 
    {
        if(head==NULL||head->next==NULL)
            return head;
        ListNode *first=head;
        ListNode *second=head->next;
        head->next=swapPairs(second->next);
        second->next=first;
        return second;  
    }
};
ListNode *createByTail()
{
    ListNode *head;
    ListNode *p1,*p2;
    int n=0,num;
    int len;
    cin>>len;
    head=NULL;
    while(n<len && cin>>num)
    {
        p1=new ListNode(num);
        n=n+1;
        if(n==1)
            head=p1;
        else
            p2->next=p1;
        p2=p1;
    }
    return head;
}
void  displayLink(ListNode *head)
{
    ListNode *p;
    p=head;
    cout<<"head-->";
    while(p!= NULL)
    {
        cout<<p->val<<"-->";
        p=p->next;
    }
    cout<<"tail\n";
}
int main()
{
    ListNode* head = createByTail();
    head=Solution().swapPairs(head);
    displayLink(head);
    return 0;
}