feat: add solution to lc problem: No.0005 (#4043)

Author: djvelimir

solution/0000-0099/0005.Longest Palindromic Substring/README.md

@@ -456,6 +456,39 @@ impl Solution {
 }
 ```
 
+#### C#
+
+```cs
+public class Solution {
+    private string s;
+    private int n;
+
+    public String LongestPalindrome(string s) {
+        this.s = s;
+        n = s.Length;
+        int start = 0, mx = 1;
+        for (int i = 0; i < n; ++i) {
+            int a = F(i, i);
+            int b = F(i, i + 1);
+            int t = Math.Max(a, b);
+            if (mx < t) {
+                mx = t;
+                start = i - ((t - 1) >> 1);
+            }
+        }
+        return s.Substring(start, start + mx);
+    }
+
+    private int F(int l, int r) {
+        while (l >= 0 && r < n && s[l] == s[r]) {
+            --l;
+            ++r;
+        }
+        return r - l - 1;
+    }
+}
+```
+
 #### PHP
 
 ```php

solution/0000-0099/0005.Longest Palindromic Substring/README_EN.md

@@ -454,6 +454,39 @@ impl Solution {
 }
 ```
 
+#### C#
+
+```cs
+public class Solution {
+    private string s;
+    private int n;
+
+    public String LongestPalindrome(string s) {
+        this.s = s;
+        n = s.Length;
+        int start = 0, mx = 1;
+        for (int i = 0; i < n; ++i) {
+            int a = F(i, i);
+            int b = F(i, i + 1);
+            int t = Math.Max(a, b);
+            if (mx < t) {
+                mx = t;
+                start = i - ((t - 1) >> 1);
+            }
+        }
+        return s.Substring(start, start + mx);
+    }
+
+    private int F(int l, int r) {
+        while (l >= 0 && r < n && s[l] == s[r]) {
+            --l;
+            ++r;
+        }
+        return r - l - 1;
+    }
+}
+```
+
 #### PHP
 
 ```php

solution/0000-0099/0005.Longest Palindromic Substring/Solution2.cs

@@ -0,0 +1,28 @@
+public class Solution {
+    private string s;
+    private int n;
+
+    public String LongestPalindrome(string s) {
+        this.s = s;
+        n = s.Length;
+        int start = 0, mx = 1;
+        for (int i = 0; i < n; ++i) {
+            int a = F(i, i);
+            int b = F(i, i + 1);
+            int t = Math.Max(a, b);
+            if (mx < t) {
+                mx = t;
+                start = i - ((t - 1) >> 1);
+            }
+        }
+        return s.Substring(start, start + mx);
+    }
+
+    private int F(int l, int r) {
+        while (l >= 0 && r < n && s[l] == s[r]) {
+            --l;
+            ++r;
+        }
+        return r - l - 1;
+    }
+}