Update 0257.二叉树的所有路径，添加C#递归

problems/0257.二叉树的所有路径.md

@@ -900,6 +900,43 @@ impl Solution {
     }
 }
 ```
+### C#
+```C#
+public IList<string> BinaryTreePaths(TreeNode root)
+{
+    List<int> path = new();
+    List<string> res = new();
+    if (root == null) return res;
+    Traversal(root, path, res);
+    return res;
+}
+public void Traversal(TreeNode node, List<int> path, List<string> res)
+{
+    path.Add(node.val);
+    if (node.left == null && node.right == null)
+    {
+        string sPath = "";
+        for (int i = 0; i < path.Count - 1; i++)
+        {
+            sPath += path[i].ToString();
+            sPath += "->";
+        }
+        sPath += path[path.Count - 1].ToString();
+        res.Add(sPath);
+        return;
+    }
+    if (node.left != null)
+    {
+        Traversal(node.left, path, res);
+        path.RemoveAt(path.Count-1);
+    }
+    if (node.right != null)
+    {
+        Traversal(node.right, path, res);
+        path.RemoveAt(path.Count-1);
+    }
+}
+```
 
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