Update 0647.回文子串.md

回文子串添加java动态规划简单版本的代码

problems/0647.回文子串.md

@@ -267,6 +267,27 @@ class Solution {
         return ans;
     }
 }
+
+```
+
+动态规划：简洁版
+```java
+class Solution {
+    public int countSubstrings(String s) {
+        boolean[][] dp = new boolean[s.length()][s.length()];
+        
+        int res = 0;
+        for (int i = s.length() - 1; i >= 0; i--) {
+            for (int j = i; j < s.length(); j++) {
+                if (s.charAt(i) == s.charAt(j) && (j - i <= 1 || dp[i + 1][j - 1])) {
+                    res++;
+                    dp[i][j] = true;
+                }
+            }
+        }
+        return res;
+    }
+}
 ```
 
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