public class JieCheng3 {    static long[] table = new long[21];    static { table[0] = 1; }      static int last = 0;    public static long factorial(int x) throws IllegalArgumentException {        if (x >= table.length)               throw new IllegalArgumentException("Overflow; x is too large.");        if (x<0)             throw new IllegalArgumentException("x must be non-negative.");        while(last < x) {            table[last + 1] = table[last] * (last + 1);            last++;        }                return table[x];    }}