Update.二叉树的递归遍历，添加C#版

problems/二叉树的递归遍历.md

@@ -565,6 +565,60 @@ impl Solution {
 }
 ```
 
+### C#
+```C#
+// 前序遍历
+public IList<int> PreorderTraversal(TreeNode root)
+{
+    var res = new List<int>();
+    if (root == null) return res;
+    Traversal(root, res);
+    return res;
+
+}
+public void Traversal(TreeNode cur, IList<int> res)
+{
+    if (cur == null) return;
+    res.Add(cur.val);
+    Traversal(cur.left, res);
+    Traversal(cur.right, res);
+}
+```
+```C#
+// 中序遍历
+public IList<int> InorderTraversal(TreeNode root)
+{
+    var res = new List<int>();
+    if (root == null) return res;
+    Traversal(root, res);
+    return res;
+}
+public void Traversal(TreeNode cur, IList<int> res)
+{
+    if (cur == null) return;
+    Traversal(cur.left, res);
+    res.Add(cur.val);
+    Traversal(cur.right, res);
+}
+```
+```C#
+// 后序遍历
+public IList<int> PostorderTraversal(TreeNode root)
+{
+    var res = new List<int>();
+    if (root == null) return res;
+    Traversal(root, res);
+    return res;
+}
+public void Traversal(TreeNode cur, IList<int> res)
+{
+    if (cur == null) return;
+    Traversal(cur.left, res);
+    Traversal(cur.right, res);
+    res.Add(cur.val);
+}
+```
+
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