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➤微信公众号：山青咏芝（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: num = "123", target = 6
Output: ["1+2+3", "1*2*3"]
Example 2:
Input: num = "232", target = 8
Output: ["2*3+2", "2+3*2"]
Example 3:
Input: num = "105", target = 5
Output: ["1*0+5","10-5"]
Example 4:
Input: num = "00", target = 0
Output: ["0+0", "0-0", "0*0"]
Example 5:
Input: num = "3456237490", target = 9191
Output: []
给定一个仅包含数字 0-9 的字符串和一个目标值，在数字之间添加二元运算符（不是一元）+、- 或 * ，返回所有能够得到目标值的表达式。
示例 1:
输入: num = "123", target = 6
输出: ["1+2+3", "1*2*3"]
示例 2:
输入: num = "232", target = 8
输出: ["2*3+2", "2+3*2"]
示例 3:
输入: num = "105", target = 5
输出: ["1*0+5","10-5"]
示例 4:
输入: num = "00", target = 0
输出: ["0+0", "0-0", "0*0"]
示例 5:
输入: num = "3456237490", target = 9191
输出: []
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 }