将双向循环链表L=(a1,a2,...,an)改造成L=(a1,a3,...,an,...,a4,a2)

typedef struct BiNode {
ElemType       data;
int            freq;
struct BiNode *prev,
*next;
} BiNode, *BiLinkList;

void ReverseEven(BiLinkList &L)
{
BiLinkList p,q,s,r;
int i = 0;
q = L;
p = L;
p = p -> next;
r = p;
while(p -> next != L){
s = p -> next;
i++;    /**i值表示当前p指针的指向**/
if(i%2 == 0){  /**将第偶数个指针移到头指针左边**/
q -> prev = p;
p -> next = q;
q = q -> prev;
p = s;            /**记录p点**/
r -> next = s;   /**将s指针指向的节点与修改后的前一节点相连接**/
s -> prev = r;
r = r -> next;
}
else    /**不符合条件，p指针往后移**/
p = p -> next;
}
p -> next = q;
q -> prev = p;
}