Add Dart solution for Valid Palindrome

dart/valid_palindrome.dart

@@ -0,0 +1,39 @@
+/*
+A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward.
+
+Given a string s, return true if it is a palindrome, or false otherwise.
+*/
+
+class Solution {
+  bool isPalindrome(String s) {
+    int left = 0;
+    int right = s.length - 1;
+
+    while (left < right) {
+      while (left < right && !isAlphaNumeric(s[left])) {
+        left++;
+      }
+
+      while (left < right && !isAlphaNumeric(s[right])) {
+        right--;
+      }
+
+      if (s[left].toLowerCase() != s[right].toLowerCase()) {
+        return false;
+      }
+
+      left++;
+      right--;
+    }
+
+    return true;
+  }
+
+  bool isAlphaNumeric(String char) {
+    final int code = char.codeUnitAt(0);
+
+    return (code >= 48 && code <= 57) ||
+        (code >= 65 && code <= 90) ||
+        (code >= 97 && code <= 122);
+  }
+}