leetcode之24. Swap Nodes in Pairs（C++实现&链表实现和整体测试）

题目：
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.
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struct ListNode{    int val;    ListNode *next;    ListNode(int x) : val(x), next(NULL) {}};class linkedlist{public:    linkedlist(): head(nullptr) {};    ~linkedlist();    void inserts(int val);    ListNode* swapPairs(ListNode* head);    void prints();//private:    ListNode *head;};void linkedlist::prints(){    if(head==nullptr)    {        cout<<"The Linkedlist is empty"<<endl;        return ;    }    cout<<head->val;    ListNode * heads=head->next;    while(heads)    {        cout<<"  ->  "<<heads->val;        heads=heads->next;    }    cout<<endl;}ListNode* linkedlist::swapPairs(ListNode* head){    if(!head || !head->next) return head;    ListNode* temp;    ListNode* new_head;    //change num1 & num2    ListNode* p1=head;    ListNode* p2=head->next;    p1->next=p2->next;    p2->next=p1;    head=p2;    ListNode* p=head->next;    while(p->next && p->next->next)    {        //init        temp=p->next;        new_head=temp->next;        p->next=new_head;        temp->next=new_head->next;        new_head->next=temp;        //update p        p=temp;    }    return head;}void linkedlist::inserts(int val){    if(nullptr==head)    {        head=new ListNode(val);    }    else    {        ListNode *heads =  head;        while(heads->next)        {            heads=heads->next;        }        heads->next=new ListNode(val);    }    cout<<"insert success!"<<endl;    return ;}int main(){    linkedlist* s = new linkedlist();    s->prints();    s->inserts(1);    s->inserts(2);    s->inserts(3);    s->inserts(4);    s->inserts(5);    s->prints();    s->head=s->swapPairs(s->head);    s->prints();    return 0;}