diff --git a/src/leetcode/string.md b/src/leetcode/string.md
index f3ad14f..b4f2951 100644
--- a/src/leetcode/string.md
+++ b/src/leetcode/string.md
@@ -1 +1,29 @@
-# String
\ No newline at end of file
+# String
++ [Valid Palindrome](#valid-palindrome)
+## Valid Palindrome
+https://leetcode.com/problems/valid-palindrome/
+```java
+class Solution {
+    private static int toLowerCase(char c) {
+        return (c >= 'A' && c <= 'Z' ? c + ('a' - 'A') : c);
+    }
+    static boolean isAlphaNumeric(char c) {
+        return  (c >= 'a' && c <= 'z') ||
+                (c >= 'A' && c <= 'Z') ||
+                (c >= '0' && c <= '9');
+    }
+    public boolean isPalindrome(String s) {
+        char[] ch = s.toCharArray();
+        int start = 0;
+        int end = ch.length - 1;
+        while (start < end) {
+            if (!isAlphaNumeric(ch[start])) start++;
+            else if (!isAlphaNumeric(ch[end])) end--;
+            else if (toLowerCase(ch[start++]) == toLowerCase(ch[end--])) {
+            } else return false;
+        }
+        return true;
+    }
+}
+
+```