[Swift]LeetCode282. 給表達式添加運算符 | Expression Add Operators

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➤微信公衆號：山青詠芝（shanqingyongzhi）
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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.

Example 1:

Input: "123", target = 6
Output: ["1+2+3", "1*2*3"] 
num =

Example 2:

Input: "232", target = 8
Output: ["2*3+2", "2+3*2"]num =

Example 3:

Input: "105", target = 5
Output: ["1*0+5","10-5"]num =

Example 4:

Input: "00", target = 0
Output: ["0+0", "0-0", "0*0"]
num =

Example 5:

Input: "3456237490", target = 9191
Output: []num =

---

給定一個僅包含數字 0-9 的字符串和一個目標值，在數字之間添加二元運算符（不是一元）+、- 或 * ，返回全部可以獲得目標值的表達式。

示例 1:

輸入: "123", target = 6
輸出: ["1+2+3", "1*2*3"] 
num =

示例 2:

輸入: "232", target = 8
輸出: ["2*3+2", "2+3*2"]num =

示例 3:

輸入: "105", target = 5
輸出: ["1*0+5","10-5"]num =

示例 4:

輸入: "00", target = 0
輸出: ["0+0", "0-0", "0*0"]
num =

示例 5:

輸入: "3456237490", target = 9191
輸出: []num =

---

1416ms

 1 class Solution {
 2     func addOperators(_ num: String, _ target: Int) -> [String] {
 3         let numArr = Array(num)
 4         var res = [String]()
 5         
 6         opeHelp(numArr, 0, 0, 0, "", &res, target)
 7         
 8         return res
 9     }
10     
11     func opeHelp(_ numArr : [Character],_ total : Int,_ last : Int , _ index : Int,_ str : String, _ res : inout [String], _ target : Int) {
12         if index == numArr.count {
13             if total == target {
14                 res.append(str)
15             }
16             return
17         }
18         
19         for i in index+1...numArr.count {
20             let tmp = String(numArr[index..<i])
21             let count = tmp.count
22             if count > 1 && numArr[index] == "0" {
23                 continue
24             }
25             
26             let n = Int(tmp)!
27             
28             if index == 0 {
29                 opeHelp(numArr, total + n, n,i, str + tmp, &res, target)
30                 continue
31             }
32             
33             opeHelp(numArr, total + n, n,i, str + "+" + tmp, &res, target)
34             opeHelp(numArr, total - n, -n,i, str + "-" + tmp, &res, target)
35             opeHelp(numArr, (total - last) + last * n, last * n, i,str + "*" + tmp, &res, target)
36         }
37         
38     }
39 }