前序遍历全部的BT，顺序插入结果数组中。Note：bisect.bisect_left, bisect.bisect_right, bisect.insort, bisect.insort_left, bisect.insort_right得牢固记忆。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def getAllElements(self, root1: TreeNode, root2: TreeNode) -> List[int]:
        self.res = []
        def traverse(root):
            if root is None:return
            bisect.insort_left(self.res, root.val)
            traverse(root.left)
            traverse(root.right)
        traverse(root1)
        traverse(root2)
        return self.res