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 -> []
1 public class Solution { 2 public IList<string> AddOperators(string num, int target) { 3 var result = new List<string>(); 4 if (num == null || num.Length == 0) return result; 5 6 DFS("", result, num, (long)target, 0, 0, 0); 7 return result; 8 } 9 10 private void DFS(string path, IList<string> result, string num, long target, int start, long eval, long multied) 11 { 12 if (start >= num.Length) 13 { 14 if (eval == target) 15 { 16 result.Add(path); 17 } 18 19 return; 20 } 21 22 for (int i = start; i < num.Length; i++) 23 { 24 if (num[start] == '0' && i > start) break; 25 26 long cur = Int64.Parse(num.Substring(start, i - start + 1)); 27 28 if (start == 0) 29 { 30 DFS(cur.ToString(), result, num, target, i + 1, cur, cur); 31 } 32 else 33 { 34 DFS(path + "+" + cur, result, num, target, i + 1, eval + cur, cur); 35 DFS(path + "-" + cur, result, num, target, i + 1, eval - cur, -cur); 36 DFS(path + "*" + cur, result, num, target, i + 1, eval - multied + multied * cur, multied * cur); 37 } 38 } 39 } 40 }