Leetcode 282: 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 -> []

public class Solution {
    public IList<string> AddOperators(string num, int target) {
        var result = new List<string>();
        if (num == null || num.Length == 0) return result;
        
        DFS("", result, num, (long)target, 0, 0, 0);
        return result;
    }
    
    private void DFS(string path, IList<string> result, string num, long target, int start, long eval, long multied)
    {
        if (start >= num.Length)
        {
            if (eval == target)
            {
                result.Add(path);
            }
            
            return;
        }
        
        for (int i = start; i < num.Length; i++)
        {
            if (num[start] == '0' && i > start) break;
            
            long cur = Int64.Parse(num.Substring(start, i - start + 1));
            
            if (start == 0)
            {
                DFS(cur.ToString(), result, num, target, i + 1, cur, cur);    
            }
            else
            {
                DFS(path + "+" + cur, result, num, target, i + 1, eval + cur, cur);
                DFS(path + "-" + cur, result, num, target, i + 1, eval - cur, -cur);
                DFS(path + "*" + cur, result, num, target, i + 1, eval - multied + multied * cur, multied * cur);
            }
        }
    }
}