feat: add cs solution to lc problem: No.0501 (#1944)

Author: dev-mauli

solution/0500-0599/0501.Find Mode in Binary Search Tree/README.md

@@ -224,6 +224,43 @@ func findMode(root *TreeNode) []int {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    private int mx;
+    private int cnt;
+    private TreeNode prev;
+    private List<int> res;
+
+    public int[] FindMode(TreeNode root) {
+        res = new List<int>();
+        Dfs(root);
+        int[] ans = new int[res.Count];
+        for (int i = 0; i < res.Count; ++i) {
+            ans[i] = res[i];
+        }
+        return ans;
+    }
+
+    private void Dfs(TreeNode root) {
+        if (root == null) {
+            return;
+        }
+        Dfs(root.left);
+        cnt = prev != null && prev.val == root.val ? cnt + 1 : 1;
+        if (cnt > mx) {
+            res = new List<int>(new int[] { root.val });
+            mx = cnt;
+        } else if (cnt == mx) {
+            res.Add(root.val);
+        }
+        prev = root;
+        Dfs(root.right);
+    }
+}
+```
+
 ### **...**
 
 ```

solution/0500-0599/0501.Find Mode in Binary Search Tree/README_EN.md

@@ -211,6 +211,43 @@ func findMode(root *TreeNode) []int {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    private int mx;
+    private int cnt;
+    private TreeNode prev;
+    private List<int> res;
+
+    public int[] FindMode(TreeNode root) {
+        res = new List<int>();
+        Dfs(root);
+        int[] ans = new int[res.Count];
+        for (int i = 0; i < res.Count; ++i) {
+            ans[i] = res[i];
+        }
+        return ans;
+    }
+
+    private void Dfs(TreeNode root) {
+        if (root == null) {
+            return;
+        }
+        Dfs(root.left);
+        cnt = prev != null && prev.val == root.val ? cnt + 1 : 1;
+        if (cnt > mx) {
+            res = new List<int>(new int[] { root.val });
+            mx = cnt;
+        } else if (cnt == mx) {
+            res.Add(root.val);
+        }
+        prev = root;
+        Dfs(root.right);
+    }
+}
+```
+
 ### **...**
 
 ```

solution/0500-0599/0501.Find Mode in Binary Search Tree/Solution.cs

@@ -0,0 +1,32 @@
+public class Solution {
+    private int mx;
+    private int cnt;
+    private TreeNode prev;
+    private List<int> res;
+    
+    public int[] FindMode(TreeNode root) {
+        res = new List<int>();
+        Dfs(root);
+        int[] ans = new int[res.Count];
+        for (int i = 0; i < res.Count; ++i) {
+            ans[i] = res[i];
+        }
+        return ans;
+    }
+    
+    private void Dfs(TreeNode root) {
+        if (root == null) {
+            return;
+        }
+        Dfs(root.left);
+        cnt = prev != null && prev.val == root.val ? cnt + 1 : 1;
+        if (cnt > mx) {
+            res = new List<int>(new int[] { root.val });
+            mx = cnt;
+        } else if (cnt == mx) {
+            res.Add(root.val);
+        }
+        prev = root;
+        Dfs(root.right);
+    }
+}