Update104.二叉树的最大深度，添加C#版

problems/0104.二叉树的最大深度.md

@@ -1022,6 +1022,61 @@ impl Solution {
         max_depth
     }
 ```
+### C#
+```C#
+// 递归法
+public int MaxDepth(TreeNode root) {
+    if(root == null) return 0;
+
+    int leftDepth = MaxDepth(root.left);
+    int rightDepth = MaxDepth(root.right);
+
+    return 1 + Math.Max(leftDepth, rightDepth);
+}
+```
+```C#
+// 前序遍历
+int result = 0;
+public int MaxDepth(TreeNode root)
+{
+    if (root == null) return result;
+    GetDepth(root, 1);
+    return result;
+}
+public void GetDepth(TreeNode root, int depth)
+{
+    result = depth > result ? depth : result;
+    if (root.left == null && root.right == null) return;
+
+    if (root.left != null)
+        GetDepth(root.left, depth + 1);
+    if (root.right != null)
+        GetDepth(root.right, depth + 1);
+    return;
+}
+```
+```C#
+// 迭代法
+public int MaxDepth(TreeNode root)
+{
+    int depth = 0;
+    Queue<TreeNode> que = new();
+    if (root == null) return depth;
+    que.Enqueue(root);
+    while (que.Count != 0)
+    {
+        int size = que.Count;
+        depth++;
+        for (int i = 0; i < size; i++)
+        {
+            var node = que.Dequeue();
+            if (node.left != null) que.Enqueue(node.left);
+            if (node.right != null) que.Enqueue(node.right);
+        }
+    }
+    return depth;
+}
+```
 
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