Biểu thức Zero (thuật toán Quay lui BackTracking)

package Advance;

import java.io.FileInputStream;
import java.util.Scanner;

public class BieuThucZero {
public static void main(String[] args) throws Exception {
new BieuThucZero().inIt();
}

int N, Result;
int key[];
int k, t;
String bieuthuc;

private void inIt() throws Exception {
Scanner sc = new Scanner(System.in);
int T;
T = sc.nextInt();
for (int test_case = 1; test_case <= T; test_case++) {
N = sc.nextInt();
key = new int[N - 1];
Result = 0;
backtrack(0);
System.out.println("#"+test_case + " " + Result);
}
}

private void backtrack(int num) {
if (num == N - 1) {
if (checkEnd() == 0) {
Result++;
}
return;
}
for (int i = 0; i < 3; i++) {
key[num] = i;
backtrack(num + 1);
}
}

private int checkEnd() {
bieuthuc = getString();
k = t = 0;
int[] num = new int[N];
int op[] = new int[N - 1];
for (int i = 0; i < bieuthuc.length(); i++) {
if (isOperand(bieuthuc.charAt(i))) {
num[k] = num[k] * 10 + (bieuthuc.charAt(i) - '0');
} else {
if (bieuthuc.charAt(i) == '+') {
op[t] = 1;
} else {
op[t] = 2;
}
t++;
k++;
num[k] = 0;
}
}
int kq = num[0];
for (int i = 0; i < t; i++) {
if (op[i] == 1) {
kq += num[i+1];
} else {
kq -= num[i+1];
}
}
return kq;
}

public boolean isOperand(char x) {
if (x - 48 >= 0 && x - 48 <= 9)
return true;
else
return false;
}

private String getString() {
String s = "";
s += 1;
for (int i = 0; i < N - 1; i++) {
if (key[i] == 0) {
s = s + (i + 2);
}
if (key[i] == 1) {
s += '+';
s = s + (i + 2);
}
if (key[i] == 2) {
s += '-';
s = s + (i + 2);
}
}
return s;
}
}