Leetcode: 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"]
"105", 5 -> ["1*0+5","10-5"]
"00", 0 -> ["0+0", "0-0", "0*0"]
"3456237490", 9191 -> []

Understand the problem:
A classic DFS problem. 


Code (Java):

public class Solution {
    public List<String> addOperators(String num, int target) {
        List<String> result = new ArrayList<>();
        
        if (num == null || num.length() == 0) {
            return result;
        }
        
        addOperatorHelper(num, 0, target, 0, 0, "", result);
        
        return result;
    }
    
    private void addOperatorHelper(String num, int start, int target, long curSum, 
                   long preNum, String curResult, List<String> result) {
        if (start == num.length() && curSum == target) {
            result.add(curResult);
            return;
        }
        
        if (start == num.length()) {
            return;
        }
        
        for (int i = start; i < num.length(); i++) {
            String curStr = num.substring(start, i + 1);
            if (curStr.length() > 1 && curStr.charAt(0) == '0') {
                break;
            }
            
            long curNum = Long.parseLong(curStr);
            
            if (curResult.isEmpty()) {
                addOperatorHelper(num, i + 1, target, curNum, curNum, curStr, result);
            } else {
                // Multiply
                addOperatorHelper(num, i + 1, target, curSum - preNum + preNum * curNum, 
                                  preNum * curNum, curResult + "*" + curNum, result);
                
                // Add
                addOperatorHelper(num, i + 1, target, curSum + curNum, curNum, 
                                  curResult + "+" + curNum, result);
                
                // Subtract
                addOperatorHelper(num, i + 1, target, curSum - curNum, -curNum, 
                                  curResult + "-" + curNum, result);
            }
        }
    }
}