Add solution to 'a lot' of problems

Easy/valid-palindrome/solution.java

@@ -0,0 +1,20 @@
+class Solution {
+    public boolean isPalindrome(String s) {
+        String t = "";
+        for (int i = 0; i < s.length(); ++i) {
+            int a = (int)s.charAt(i);
+            if (a >= (int)'A' && a <= (int)'Z') {
+                t += (char)(a - (int)'A' + (int)'a');
+            } else if (a >= (int)'a' && a <= (int)'z') {
+                t += (char)a;
+            } else if (a >= (int)'0' && a <= (int)'9') {
+                t += (char)a;
+            }
+        }
+
+        StringBuilder ob = new StringBuilder(t);
+        String r = ob.reverse().toString();
+
+        return t.compareTo(r) == 0;
+    }
+}