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: 

"123", 6 -> ["1+2+3", "1*2*3"] 
"232", 8 -> ["2*3+2", "2+3*2"]
"00", 0 -> ["0+0", "0-0", "0*0"]
"3456237490", 9191 -> []

Credits:
Special thanks to @davidtan1890 for adding this problem and creating all test cases.

public class Solution {
    public List<String> addOperators(String num, int target) {
        List<String> res = new ArrayList<String>();
        if (num == null || num.length() == 0) {
            return res;
        }
        helper(num, target, "", 0, 0, 0, res);
        return res;
    }
   
    private void helper(String num, int target, String expression, int pos, long total, long lastVal, List<String> res) {
        if (pos == num.length()) {
            if (total == target) {
                res.add(expression);
            }
        }
        for (int i = pos; i < num.length(); i++) {
            if (i != pos && num.charAt(pos) == '0') {
                break;
            }
            long val = Long.parseLong(num.substring(pos, i + 1));
            if (pos == 0) {
                helper(num, target, expression + num.substring(pos, i + 1), i + 1, total + val, val, res);
            } else {
                helper(num, target, expression + "+" + val, i + 1, total + val, val, res);
                helper(num, target, expression + "-" + val, i + 1, total - val, -val, res);
                helper(num, target, expression + "*" + val, i + 1, total - lastVal + val * lastVal, val * lastVal, res);
            }
        }
    }
}