王道书P40 T17（循环双链表实现）

#include 
#include 

typedef struct DRNode{
    int data;
    struct DRNode *next,*pre;
}DRNode,*DRLinkList;

bool IsSymmetry(DRLinkList L){
    DRLinkList p = L -> next;
    DRLinkList q = L -> pre;
    while(1){
        p = p -> next;
        q = q -> pre;
        if(p == q || p -> next == q)
            return true;
    }
    return false;
}