字符串排序----三向字符串快速排序

public class Quick3string {
    private static int charAt(String s, int d) {
        if(d<s.length())return s.charAt(d);
        else return -1;
    }
    public static void sort(String[] a) { sort(a,0,a.length-1,0); }
	
    public static void sort(String[] a,int lo, int hi, int d) {
        if(hi<=lo)	return;
        int lt = lo, gt = hi;
        int v = charAt(a[lo], d);
        int i = lo+1;
        while(i<=gt) {
            int t = charAt(a[lo],d);
            if(t<v)	exch(a,lt++,i++);
            else if(t>v)	exch(a,i,gt--);
            else i++;
        }
        sort(a,lo,lt-1,d);
        if(v>=0) sort(a,lt,gt,d+1);
        sort(a,gt+1,hi,d);
    }
}