Expression Add Operators

Given a string that contains only digits 0-9 and a target value, return all possibilities to add binary operators (not unary) +, -, or * between the digits so they evaluate to the target value.
Examples:
1. "123", 6 -> ["1+2+3", "123"]
2. "232", 8 -> ["23+2", "2+32"]
3. "105", 5 -> ["10+5","10-5"]
4. "00", 0 -> ["0+0", "0-0", "00"]
5. "3456237490", 9191 -> []

Solution:
1. public class Solution {
2. public List addOperators(String num, int target) {
3. List res = new ArrayList<>();
4. 5. if (num == null || num.length() == 0)
6. return res;
7. 8. helper(num, target, 0, 0, 0, "", res);
9. 10. return res;
11. }
12. 13. public void helper(String num, int target, int index, long total, long last, String sol, List res){
14. if (index == num.length()) {
15. if (target == total)
16. res.add(sol);
17. 18. return;
19. }
20. 21. for (int i = index; i < num.length(); i++) {
22. // for example, input is "105", we don't need answer like "105"
23. if (i > index && num.charAt(index) == '0')
24. break;
25. 26. long curr = Long.parseLong(num.substring(index, i + 1));
27. 28. if (index == 0) {
29. // 第一个数
30. helper(num, target, i + 1, curr, curr, sol + curr, res);
31. } else {
32. // 不是第一个数, 我们可以添加运算符了
33. helper(num, target, i + 1, total + curr, curr, sol + "+" + curr, res);
34. helper(num, target, i + 1, total - curr, -curr, sol + "-" + curr, res);
35. helper(num, target, i + 1, total - last + last * curr, last * curr, sol + "" + curr, res);
36. }
37. }
38. }
39. }