feat: add C# solution to lc problem: No.0621 (doocs#861)

No.0621.Task Scheduler

Author: wyh317

solution/0600-0699/0621.Task Scheduler/README.md

@@ -158,6 +158,26 @@ func max(a, b int) int {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public int LeastInterval(char[] tasks, int n) {
+        int[] cnt = new int[26];
+        int x = 0;
+        foreach (char c in tasks) {
+            cnt[c - 'A']++;
+            x = Math.Max(x, cnt[c - 'A']);
+        }
+        int s = 0;
+        foreach (int v in cnt) {
+            s = v == x ? s + 1 : s;
+        }
+        return Math.Max(tasks.Length, (x - 1) * (n + 1) + s);
+    }
+}
+```
+
 ### **...**
 
 ```

solution/0600-0699/0621.Task Scheduler/README_EN.md

@@ -141,6 +141,26 @@ func max(a, b int) int {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public int LeastInterval(char[] tasks, int n) {
+        int[] cnt = new int[26];
+        int x = 0;
+        foreach (char c in tasks) {
+            cnt[c - 'A']++;
+            x = Math.Max(x, cnt[c - 'A']);
+        }
+        int s = 0;
+        foreach (int v in cnt) {
+            s = v == x ? s + 1 : s;
+        }
+        return Math.Max(tasks.Length, (x - 1) * (n + 1) + s);
+    }
+}
+```
+
 ### **...**
 
 ```

solution/0600-0699/0621.Task Scheduler/Solution.cs

@@ -0,0 +1,15 @@
+public class Solution {
+    public int LeastInterval(char[] tasks, int n) {
+        int[] cnt = new int[26];
+        int x = 0;
+        foreach (char c in tasks) {
+            cnt[c - 'A']++;
+            x = Math.Max(x, cnt[c - 'A']);
+        }
+        int s = 0;
+        foreach (int v in cnt) {
+            s = v == x ? s + 1 : s;
+        }
+        return Math.Max(tasks.Length, (x - 1) * (n + 1) + s);
+    }
+}