[Swift]LeetCode640. 求解方程 | Solve the Equation

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➤微信公衆號：山青詠芝（shanqingyongzhi）
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➤GitHub地址：https://github.com/strengthen/LeetCode
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Solve a given equation and return the value of x in the form of string "x=#value". The equation contains only '+', '-' operation, the variable x and its coefficient.

If there is no solution for the equation, return "No solution".

If there are infinite solutions for the equation, return "Infinite solutions".

If there is exactly one solution for the equation, we ensure that the value of x is an integer.

Example 1:

Input: "x+5-3+x=6+x-2"
Output: "x=2" 

Example 2:

Input: "x=x"
Output: "Infinite solutions" 

Example 3:

Input: "2x=x"
Output: "x=0" 

Example 4:

Input: "2x+3x-6x=x+2"
Output: "x=-1" 

Example 5:

Input: "x=x+2"
Output: "No solution"

---

求解一個給定的方程，將x以字符串"x=#value"的形式返回。該方程僅包含'+'，' - '操做，變量 x 和其對應係數。

若是方程沒有解，請返回「No solution」。

若是方程有無限解，則返回「Infinite solutions」。

若是方程中只有一個解，要保證返回值 x 是一個整數。

示例 1：

輸入: "x+5-3+x=6+x-2"
輸出: "x=2"

示例 2:

輸入: "x=x"
輸出: "Infinite solutions"

示例 3:

輸入: "2x=x"
輸出: "x=0"

示例 4:

輸入: "2x+3x-6x=x+2"
輸出: "x=-1"

示例 5:

輸入: "x=x+2"
輸出: "No solution"

---

Runtime: 4 ms

Memory Usage: 20 MB

 1 class Solution {
 2     func solveEquation(_ equation: String) -> String {
 3         var arr:[Character] = Array(equation)
 4         var n:Int = equation.count
 5         var a:Int = 0
 6         var b:Int = 0
 7         var sign:Int = 1
 8         var j:Int = 0
 9         for i in 0..<n
10         {
11             if arr[i] == "+" || arr[i] == "-"
12             {
13                 if i > j
14                 {
15                     b += Int(equation.subString(j, i - j))! * sign
16                 }
17                 j = i
18             }
19             else if arr[i] == "x"
20             {
21                 if i == j || arr[i - 1] == "+"
22                 {
23                     a += sign
24                 }
25                 else if arr[i - 1] == "-"
26                 {
27                     a -= sign
28                 }
29                 else
30                 {
31                     a += Int(equation.subString(j, i - j))! * sign
32                 }
33                 j = i + 1
34             }
35             else if arr[i] == "="
36             {
37                 if i > j
38                 {
39                     b += Int(equation.subString(j, i - j))! * sign
40                 }
41                 sign = -1
42                 j = i + 1
43             }
44         }
45         if j < n {b += Int(equation.subString(j))! * sign}
46         if a == 0 && a == b {return "Infinite solutions"}
47         if a == 0 && a != b {return "No solution"}
48         return "x=" + String(-b / a)
49     }
50 }
51 
52 extension String {    
53     // 截取字符串：指定索引和字符數
54     // - begin: 開始截取處索引
55     // - count: 截取的字符數量
56     func subString(_ begin:Int,_ count:Int) -> String {
57         let start = self.index(self.startIndex, offsetBy: max(0, begin))
58         let end = self.index(self.startIndex, offsetBy:  min(self.count, begin + count))
59         return String(self[start..<end]) 
60     }
61     
62     // 截取字符串：從index到結束處
63     // - Parameter index: 開始索引
64     // - Returns: 子字符串
65     func subString(_ index: Int) -> String {
66         let theIndex = self.index(self.endIndex, offsetBy: index - self.count)
67         return String(self[theIndex..<endIndex])
68     }
69 }

---

8ms

 1 class Solution {
 2     func solveEquation(_ equation: String) -> String {
 3         let parts = equation.components(separatedBy: "=")
 4         let res1 = resolve(parts[0]), res2 = resolve(parts[1])
 5         let n = (res2.1 - res1.1)
 6         let x = (res1.0 - res2.0)
 7         if n == 0 && x == 0 { return "Infinite solutions" }
 8         if x == 0 { return "No solution" }
 9         return "x=\(n/x)"
10     }
11     
12     private func resolve(_ e: String) -> (Int, Int) {
13         var x = 0, n = 0
14         var num = 0
15         var isX = false
16         var sign = 1
17         var ca = Array(e)
18         ca.append("+")
19         for i in 0..<ca.count {
20             let c = ca[i]
21             if let digit = Int(String(c)) {
22                 num *= 10
23                 num += digit
24             } else if c == "+" || c == "-" {
25                 if isX {
26                     if i <= 1 || ca[i-2] != "0" {
27                         num = num == 0 ? 1 : num
28                     }
29                     x += num * sign
30                 } else {
31                     n += num * sign
32                 }
33                 sign = c == "+" ? 1 : -1
34                 num = 0
35                 isX = false
36             } else if c == "x" {
37                 isX = true
38             }
39         }
40         return (x, n)
41     }
42 }