add solution of problem 125: valid palindrome

Author: ppulse

ValidPalindrome125/README.md

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+Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
+
+For example,
+`"A man, a plan, a canal: Panama"` is a palindrome.
+`"race a car" is *not* a palindrome.
+
+Note:
+Have you consider that the string might be empty? This is a good question to ask during an interview.

ValidPalindrome125/Solution.java

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+public class Solution {
+    public boolean isPalindrome(String s) {
+        if (s == null)
+            return false;
+        if (s.length() == 0)
+            return true;
+        
+        String sLower = s.toLowerCase();
+        
+        int left = 0;
+        int right = sLower.length() - 1;
+        
+        while (left <= right) {
+            while (left <= right && !((sLower.charAt(left) >= 'a' && sLower.charAt(left) <= 'z') 
+                   || (sLower.charAt(left) >= '0' && sLower.charAt(left) <= '9'))) {
+                left++;
+            }
+            
+            while (left <= right && !((sLower.charAt(right) >= 'a' && sLower.charAt(right) <= 'z') 
+                   || (sLower.charAt(right) >= '0' && sLower.charAt(right) <= '9'))) {
+                right--;
+            }
+            
+            if (left <= right && sLower.charAt(left) != sLower.charAt(right))
+                return false;
+            
+            left++;
+            right--;
+        }
+        
+        return true;
+    }
+}