Update.统一迭代法，添加C#版

problems/二叉树的统一迭代法.md

@@ -741,6 +741,99 @@ impl Solution{
     }
 }
 ```
+### C#
+```C#
+// 前序遍历
+public IList<int> PreorderTraversal(TreeNode root)
+{
+    var res = new List<int>();
+    var st = new Stack<TreeNode>();
+    if (root == null) return res;
+    st.Push(root);
+    while (st.Count != 0)
+    {
+        var node = st.Peek();
+        if (node == null)
+        {
+            st.Pop();
+            node = st.Peek();
+            st.Pop();
+            res.Add(node.val);
+        }
+        else
+        {
+            st.Pop();
+            if (node.right != null) st.Push(node.right);
+            if (node.left != null) st.Push(node.left);
+            st.Push(node);
+            st.Push(null);
+        }
+    }
+    return res;
+}
+```
+```C#
+// 中序遍历
+public IList<int> InorderTraversal(TreeNode root)
+{
+    var res = new List<int>();
+    var st = new Stack<TreeNode>();
+    if (root == null) return res;
+    st.Push(root);
+    while (st.Count != 0)
+    {
+        var node = st.Peek();
+        if (node == null)
+        {
+            st.Pop();
+            node = st.Peek();
+            st.Pop();
+            res.Add(node.val);
+        }
+        else
+        {
+            st.Pop();
+            if (node.right != null) st.Push(node.right);
+            st.Push(node);
+            st.Push(null);
+            if (node.left != null) st.Push(node.left);
+        }
+    }
+    return res;
+}
+```
+
+```C#
+// 后序遍历
+public IList<int> PostorderTraversal(TreeNode root)
+{
+    var res = new List<int>();
+    var st = new Stack<TreeNode>();
+    if (root == null) return res;
+    st.Push(root);
+    while (st.Count != 0)
+    {
+        var node = st.Peek();
+        if (node == null)
+        {
+            st.Pop();
+            node = st.Peek();
+            st.Pop();
+            res.Add(node.val);
+        }
+        else
+        {
+            st.Pop();
+            if (node.left != null) st.Push(node.left);
+            if (node.right != null) st.Push(node.right);
+            st.Push(node);
+            st.Push(null);
+        }
+    }
+    res.Reverse(0, res.Count);
+    return res;
+}
+```
 
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