add BFS recursion version for 104

Author: Li Li

Algorithms Basics/Chapter 3. Binary Tree/104. Maximum Depth of Binary Tree.cs

@@ -0,0 +1,27 @@
+// recursion
+public class Solution {
+    public int MaxDepth(TreeNode root) {
+        if (root == null) return 0;
+        return 1 + Math.Max(MaxDepth(root.left), MaxDepth(root.right));
+    }
+}
+
+// BFS interation
+public class SolutionBFS {
+    public int MaxDepth(TreeNode root) {
+        if (root == null) return 0;
+        Queue<TreeNode> q = new Queue<TreeNode>();
+        q.Enqueue(root);
+        int maxDepth = 0;
+        while (q.Count > 0) {
+            int size = q.Count;
+            while (size-- > 0) {
+                TreeNode node = q.Dequeue();
+                if (node.left != null) q.Enqueue(node.left);
+                if (node.right != null) q.Enqueue(node.right);
+            }
+            maxDepth++;
+        }
+        return maxDepth;
+    }
+}