Merge pull request soapyigu#118 from soapyigu/String

[String] Add solution to Valid Palindrome

String/ValidPalindrome.swift

@@ -0,0 +1,43 @@
+/**
+ * Question Link: https://leetcode.com/problems/valid-palindrome/
+ * Primary idea: Two Pointers, compare left and right until they meet
+ * 
+ * Note: ask interviewer if digit matters
+ * Time Complexity: O(n), Space Complexity: O(n)
+ *
+ */
+
+class ValidPalindrome {
+    func isPalindrome(_ s: String) -> Bool {
+        let chars = Array(s.lowercased().characters)
+
+        var left = 0
+        var right = chars.count - 1
+
+        while left < right {
+            while left < right && !isAlpha(chars[left]) {
+                left += 1
+            }
+            while left < right && !isAlpha(chars[right]) {
+                right -= 1
+            }
+
+            if chars[left] != chars[right] {
+                return false
+            } else {
+                left += 1
+                right -= 1
+            }
+        }
+
+        return true
+    }
+
+    private func isAlpha(_ char: Character) -> Bool {
+        guard let char = String(char).unicodeScalars.first else {
+            fatalError("Character is invalid")
+        }
+
+        return CharacterSet.letters.contains(char) || CharacterSet.decimalDigits.contains(char)
+    }
+}