diff --git a/problems/0005.最长回文子串.md b/problems/0005.最长回文子串.md
index 62c2cbde..4d48f184 100644
--- a/problems/0005.最长回文子串.md
+++ b/problems/0005.最长回文子串.md
@@ -615,6 +615,68 @@ 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);
+    }
+}
+```
+
+雙指針：
+```C#
+public class Solution {
+    int maxlenth = 0;
+    int left = 0;
+    int right = 0;
+    
+    public string LongestPalindrome(string s) {
+         int result = 0;
+        for (int i = 0; i < s.Length; i++) {
+            extend(s, i, i, s.Length); // 以i為中心
+            extend(s, i, i + 1, s.Length); // 以i和i+1為中心
+        }
+        return s.Substring(left, maxlenth);
+    }
+    
+    private void extend(string s, int i, int j, int n) {
+        while (i >= 0 && j < n && s[i] == s[j]) {
+            if (j - i + 1 > maxlenth) {
+                left = i;
+                right = j;
+                maxlenth = j - i + 1;
+            }
+            i--;
+            j++;
+        }
+    }
+}
+```
+
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