实现一个双向循环链表，每个结点中除有prior，data和next三个域之外，还增设了一个访问频度域freq。在链表被起用之前，频度域freq的值均初始化为零，而每当对链表进行一次LOCATE(L,x)的操作后，被访问的结点（即元素值等于x的结点）中的频度域freq的值便增1，同时调整链表中结点之间的次序，使其按访问频度非递增的次序顺序排列，以便始终保持被频繁访问的结点总是靠近表头结点。-Circular linked list to implement a two-way, each node in addition to prior, data and next 3 fields but also added a visit to the frequency domain freq. He was appointed in the list before the freq the frequency domain values are initialized to zero, but when the list was a LOCATE (L, x) after the operation, was visited node (ie element value is equal to the node x ) in the frequency domain freq will increase the value of 1, while modifying the list order between nodes, so that access to the frequency according to the order of non-increasing order, so always keep frequently accessed nodes are always close to the table head node.