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):

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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);             }         }     } }