143. Reorder List

先将后len／2的链表节点翻转，再依次插入前半段链表中

class Solution {
public:
    void reorderList(ListNode* head) {
        if(head==NULL||head->next==NULL)
            return;
        
        int len=0;
        ListNode* cur=head;
        while(cur)
        {
            cur=cur->next;
            len++;
        }
        if(len%2==0)
            len/=2;
        else
            len=len/2+1;
        
        cur=head;
        for(int i=0;i<len-1;i++)
            cur=cur->next;
        ListNode* Head=cur->next,*pre=Head->next,*nx=NULL;
        cur->next=NULL;
        Head->next=NULL;
        while(pre)
        {
            nx=pre->next;
            pre->next=Head;
            Head=pre;
            pre=nx;
        }
        
        cur=head;
        pre=Head;
        while(pre)
        {
            nx=pre->next;
            pre->next=cur->next;
            cur->next=pre;
            cur=pre->next;
            pre=nx;
        }
    }
    
};

应用快慢指针

class Solution {
public:
    void reorderList(ListNode* head) {
        if(head==NULL||head->next==NULL)
            return;
        ListNode* fast=head,*slow=head,*p=NULL,*q=NULL;
        while(fast->next&&fast->next->next)
        {
            fast=fast->next->next;
            slow=slow->next;
        }
        
        fast=slow->next;
        slow->next=NULL;
        p=fast,q=fast->next;
        fast->next=NULL;
        while(q)
        {
            ListNode* t=q->next;
            q->next=p;
            p=q;
            q=t;
        }
        q=head;
        while(p&&q)
        {
            ListNode* l=p->next,*r=q->next;
            p->next=q->next;
            q->next=p;
            p=l;
            q=r;
        }
    }
    
};