diff --git a/src/leetcode/string.md b/src/leetcode/string.md
index f3ad14f..76adcc4 100644
--- a/src/leetcode/string.md
+++ b/src/leetcode/string.md
@@ -1 +1,31 @@
-# String
\ No newline at end of file
+# String
++ [Longest Palindromic Substring](#longest-palindromic-substring)
+## Longest Palindromic Substring
+https://leetcode.com/problems/longest-palindromic-substring/
+```java
+class Solution {
+    public String longestPalindrome(String s) {
+        if (s == null || s.length() < 1) return "";
+        int start = 0, end = 0;
+        for (int i = 0; i < s.length(); i++) {
+            int len1 = Center(s, i, i);
+            int len2 = Center(s, i, i + 1);
+            int len = Math.max(len1, len2);
+            if (len > end - start) {
+                start = i - (len - 1) / 2;
+                end = i + len / 2;
+            }
+        }
+        return s.substring(start, end + 1);
+    }
+    private Center(String s, int left, int right) {
+        int _left = left, _right = right;
+        while (_left >= 0 && _right < s.length() && s.charAt(_left) == s.charAt(_right)) {
+            _left--;
+            _right++;
+        }
+        return _right - _left - 1;
+    }
+}
+
+```