简单的数三退一算法。对于一个给定数目的数组，首尾相接。从第一个开始，数到三时，其位置的节点被去掉。依次每次数三个数，求出最后一个没被去掉的数-Mathematics simple retreat one algorithm. For a given number of the array, both phase. From the first start数到三, the location of the node is removed. Each time followed by the thre