Update valid-palindrome.cpp

Author: kamyu104

C++/valid-palindrome.cpp

@@ -28,12 +28,12 @@ class Solution2 {
     bool isPalindrome(string s) {
         auto left = s.begin();
         auto right = prev(s.end());
-        for(; left < right;) { 
-            if(!isalnum(*left)) {
+        while (left < right) { 
+            if (!isalnum(*left)) {
                 ++left;
-            } else if(!isalnum(*right)) {
+            } else if (!isalnum(*right)) {
                 --right; 
-            } else if(tolower(*left) != tolower(*right)) {
+            } else if (tolower(*left) != tolower(*right)) {
                 return false;
             } else {
                 ++left, --right;