feat: add swift implementation to lcof problem: No.20 (#2879)

Author: klever34

lcof/面试题20. 表示数值的字符串/README.md

@@ -329,6 +329,59 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    func isNumber(_ s: String) -> Bool {
+        let chars = Array(s)
+        var i = 0, j = chars.count - 1
+
+        // Trim leading spaces
+        while i <= j && chars[i] == " " {
+            i += 1
+        }
+        // Trim trailing spaces
+        while i <= j && chars[j] == " " {
+            j -= 1
+        }
+        if i > j {
+            return false
+        }
+
+        var digit = false
+        var dot = false
+        var e = false
+
+        while i <= j {
+            let char = chars[i]
+            if char == "+" || char == "-" {
+                if i > 0 && chars[i - 1] != " " && chars[i - 1] != "e" && chars[i - 1] != "E" {
+                    return false
+                }
+            } else if char.isWholeNumber {
+                digit = true
+            } else if char == "." {
+                if dot || e {
+                    return false
+                }
+                dot = true
+            } else if char == "e" || char == "E" {
+                if !digit || e {
+                    return false
+                }
+                e = true
+                digit = false
+            } else {
+                return false
+            }
+            i += 1
+        }
+        return digit
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof/面试题20. 表示数值的字符串/Solution.swift

@@ -0,0 +1,48 @@
+class Solution {
+    func isNumber(_ s: String) -> Bool {
+        let chars = Array(s)
+        var i = 0, j = chars.count - 1
+
+        // Trim leading spaces
+        while i <= j && chars[i] == " " {
+            i += 1
+        }
+        // Trim trailing spaces
+        while i <= j && chars[j] == " " {
+            j -= 1
+        }
+        if i > j {
+            return false
+        }
+
+        var digit = false
+        var dot = false
+        var e = false
+
+        while i <= j {
+            let char = chars[i]
+            if char == "+" || char == "-" {
+                if i > 0 && chars[i - 1] != " " && chars[i - 1] != "e" && chars[i - 1] != "E" {
+                    return false
+                }
+            } else if char.isWholeNumber {
+                digit = true
+            } else if char == "." {
+                if dot || e {
+                    return false
+                }
+                dot = true
+            } else if char == "e" || char == "E" {
+                if !digit || e {
+                    return false
+                }
+                e = true
+                digit = false
+            } else {
+                return false
+            }
+            i += 1
+        }
+        return digit
+    }
+}