feat: add swift implementation to lcof2 problem: No.018 (#3006)

---------

Co-authored-by: Libin YANG <contact@yanglibin.info>

Author: klever34

lcof2/剑指 Offer II 018. 有效的回文/README.md

@@ -206,6 +206,35 @@ impl Solution {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    func isPalindrome(_ s: String) -> Bool {
+        var i = s.startIndex
+        var j = s.index(before: s.endIndex)
+
+        while i < j {
+            while i < j && !s[i].isLetter && !s[i].isNumber {
+                i = s.index(after: i)
+            }
+            while i < j && !s[j].isLetter && !s[j].isNumber {
+                j = s.index(before: j)
+            }
+            if i >= j {
+                break
+            }
+            if s[i].lowercased() != s[j].lowercased() {
+                return false
+            }
+            i = s.index(after: i)
+            j = s.index(before: j)
+        }
+        return true
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof2/剑指 Offer II 018. 有效的回文/Solution.swift

@@ -0,0 +1,24 @@
+class Solution {
+    func isPalindrome(_ s: String) -> Bool {
+        var i = s.startIndex
+        var j = s.index(before: s.endIndex)
+
+        while i < j {
+            while i < j && !s[i].isLetter && !s[i].isNumber {
+                i = s.index(after: i)
+            }
+            while i < j && !s[j].isLetter && !s[j].isNumber {
+                j = s.index(before: j)
+            }
+            if i >= j {
+                break
+            }
+            if s[i].lowercased() != s[j].lowercased() {
+                return false
+            }
+            i = s.index(after: i)
+            j = s.index(before: j)
+        }
+        return true
+    }
+}