Expression Add Operators LeetCode solution in Java

1
class Solution {
2
  String s;
3
  List<String> result;
4
  int target;
5

6
  public void operator(int i, int prev, long prod, long mid, String exp, List<Long> l) {
7
    if (i == l.size()) {
8
      if (mid + prod == target) result.add(exp);
9
      return;
10
    }
11
    if (prev == -1) {
12
      operator(i + 1, 0, -1 * l.get(i) * l.get(i - 1), mid + l.get(i - 1), exp + "*" + l.get(i), l);
13
    } else if (prev == 1) {
14
      operator(i + 1, 0, l.get(i) * l.get(i - 1), mid - l.get(i - 1), exp + "*" + l.get(i), l);
15
    } else {
16
      operator(i + 1, 0, prod * l.get(i), mid, exp + "*" + l.get(i), l);
17
    }
18
    operator(i + 1, -1, 0, mid + prod - l.get(i), exp + "-" + l.get(i), l);
19
    operator(i + 1, 1, 0, mid + prod + l.get(i), exp + "+" + l.get(i), l);
20
  }
21

22
  public void rec(int in, List<Long> l) {
23
    if (in == s.length()) {
24
      operator(1, 1, 0, l.get(0), l.get(0) + "", l);
25
      return;
26
    }
27
    if (s.charAt(in) == '0') {
28
      l.add(0L);
29
      rec(in + 1, l);
30
      l.remove(l.size() - 1);
31
    } else {
32
      for (int i = in; i < s.length(); i++) {
33
        l.add(Long.parseLong(s.substring(in, i + 1)));
34
        rec(i + 1, l);
35
        l.remove(l.size() - 1);
36
      }
37
    }
38
  }
39

40
  public List<String> addOperators(String num, int target) {
41
    result = new ArrayList<>();
42
    this.s = num;
43
    this.target = target;
44
    rec(0, new ArrayList<>(30));
45
    return result;
46
  }
47
}