diff --git a/problems/0005.最长回文子串.md b/problems/0005.最长回文子串.md
index 62c2cbde..84d8d835 100644
--- a/problems/0005.最长回文子串.md
+++ b/problems/0005.最长回文子串.md
@@ -615,6 +615,38 @@ char * longestPalindrome(char * s){
 }
 ```
 
+C#：
+
+動態規則：
+```c#
+public class Solution {
+    
+    public string LongestPalindrome(string s) {
+        bool[,] dp = new bool[s.Length, s.Length];
+        int maxlenth = 0;
+        int left = 0;
+        int right = 0;
+        for(int i = s.Length-1 ; i>=0; i--){
+            for(int j = i; j <s.Length;j++){
+                if(s[i] == s[j]){
+                    if(j - i <= 1){ // 情况一和情况二
+                        dp[i, j] = true;
+                    }else if( dp[i+1, j-1] ){ // 情况三
+                        dp[i, j] = true;
+                    }
+                }
+                if(dp[i, j] && j-i+1 > maxlenth){
+                    maxlenth = j-i+1;
+                    left = i;
+                    right = j;
+                }
+            }
+        }
+        return s.Substring(left, maxlenth);
+    }
+}
+```
+
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