增加0005.最長回文子串C# 雙指針的版本

problems/0005.最长回文子串.md

@@ -647,6 +647,36 @@ public class Solution {
 }
 ```
 
+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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