LeetCode-Reorder List

Given a singly linked list L: L₀→L₁→…→L_n-1→L_n,
reorder it to: L₀→L_n→L₁→L_n-1→L₂→L_n-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}.

确定链表长度后将后半段反序再与前半段合并就可以了，记住前后两段一定要断开不然会产生环

/**
 * 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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(head==NULL)return;
        if(head->next==NULL)return;
        int count=0;
        ListNode* ptr=head;
        while(ptr!=NULL){
            count++;
            ptr=ptr->next;
        }
        int len;
        if(count%2==0)len=count/2;
        else len=count/2+1;
        ptr=head;
        for(int i=0;i<len-1;i++)ptr=ptr->next;
        ListNode* tmp;
        ListNode* ptr2=ptr->next;
        ptr->next=NULL;
        ptr=ptr2;
        ptr2=ptr->next;
        while(ptr2!=NULL){
            tmp=ptr2->next;
            ptr2->next=ptr;
            ptr=ptr2;
            ptr2=tmp;
        }
        ptr2=head;
        len=count/2;
        for(int i=0;i<len;i++){
            tmp=ptr2->next;
            ptr2->next=ptr;
            ptr2=tmp;
            tmp=ptr->next;
            ptr->next=ptr2;
            ptr=tmp;
        }
    }
};