Merge pull request #2512 from gggxxxx/XiongGu-branch

Add dfs function execution in the tree traversal (Python)

Author: youngyangyang04

problems/二叉树的递归遍历.md

@@ -189,7 +189,8 @@ class Solution:
             res.append(node.val)
             dfs(node.left)
             dfs(node.right)
-        
+
+        dfs(root)
         return res
 
 ```
@@ -206,7 +207,8 @@ class Solution:
             dfs(node.left)
             res.append(node.val)
             dfs(node.right)
-        
+            
+        dfs(root)
         return res
 ```
 ```python
@@ -224,7 +226,8 @@ class Solution:
             dfs(node.left)
             dfs(node.right)
             res.append(node.val)
-        
+
+        dfs(root)
         return res
 ```