https://leetcode.cn/problems/merge-two-binary-trees/
    需要注意的点

    • 前序遍历

      1. # Definition for a binary tree node.
      2. # class TreeNode:
      3. #     def __init__(self, val=0, left=None, right=None):
      4. #         self.val = val
      5. #         self.left = left
      6. #         self.right = right
      7. class Solution:
      8.   def mergeTrees(self, root1: TreeNode, root2: TreeNode) -> TreeNode:
      10.       if not root1:
      11.           return root2
      12.       if not root2:
      13.           return root1
      15.       merge = TreeNode(root1.val+root2.val)
      16.       merge.left = self.mergeTrees(root1.left, root2.left)
      17.       merge.right = self.mergeTrees(root1.right, root2.right)
      18.       return merge