leetcode143 Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    void reorderList(ListNode* head) {
        //分段
        ListNode *p1,*p2;
        p1=head;
        p2=head;
        if(!p2)
            return;
        p2=p2->next;
        
        if(!p2)
            return;
            
        while(p1&&p1->next&&p2&&p2->next)
        {
            p1=p1->next;
            p2=p2->next;
            if(p2)
                p2=p2->next;
        }
        if(p1)
        {
            p2=p1->next;
            p1->next=NULL;
        }
        p1=head;
        
        if(p1==p2)
            return;
        
        //反转
        ListNode *tnode=p2;
        p2=p2->next;
        tnode->next=NULL;
        
        while(p2)
        {
            ListNode *temp=p2;
            p2=p2->next;
            temp->next=tnode;
            tnode=temp;
        }
        
        //连接
        ListNode *t=tnode;
        
        while(p1)
        {
            
            if(t)
            {
                ListNode *p12=p1;
                p1=p1->next;
                ListNode *t2=t;
                t=t->next;
                t2->next=p12->next;
                p12->next=t2;
            }
            else
                break;
        }
    }
};

note:反转时，42要放在43前面；