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}.

#include<stdio.h>
#include<stdlib.h>

typedef struct ListNode{
    int val;
    struct ListNode *next;
}ListNode;

void reorderList(ListNode *head) {
    ListNode *p;
    int i,j,cnt=0;
    for(p=head;p!=NULL;p=p->next) cnt++;
    ListNode *arr[cnt];
    if(head==NULL || head->next==NULL) return;
    for(i=0,p=head;p!=NULL;p=p->next,i++){
        arr[i]=p;
    }
    //for(i=0;i<cnt;i++) printf("%d ",arr[i]->val);
    for(i=0,j=cnt-1;i<j;i++,j--){
        arr[i]->next=arr[j];
        arr[j]->next=arr[i+1];
    }
    arr[i]->next=NULL;
    //for(p=head;p!=NULL;p=p->next) printf("%d ",p->val);
}

ListNode *createList(){
    ListNode *head,*p1,*p2;
    int data,cnt=0;
    head=NULL;
    p1=p2=(ListNode *)malloc(sizeof(ListNode));
    scanf("%d",&data);
    p1->val=data;
    while(data!=0){
        cnt++;
        if(cnt==1){
            head=p1;
        }
        else{
            p2->next=p1;
        }
        p2=p1;
        p1=(ListNode *)malloc(sizeof(ListNode));
        scanf("%d",&data);
        p1->val=data;
    }
    p2->next=NULL;
    return head;
}

void main(){
    ListNode *head,*p;
    head=createList();
    for(p=head;p!=NULL;p=p->next) printf("%d ",p->val);
    printf("\n");
    reorderList(head);
}

Reorder List