本题注意点：python二叉树类、类递归函数

import numpy as np

class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None
    def CreatTree(self, level_order, root):
        if root >= len(level_order) or level_order[root] == -1:
            return None
        else:
            t = TreeNode(level_order[root])
            t.left = self.CreatTree(level_order, root * 2 + 1)
            t.right = self.CreatTree(level_order, root * 2 + 2)
            return t
    def PRINT(self):
        if self:
            print(self.val, end='')
            if self.left:   self.left.PRINT()
            if self.right:  self.right.PRINT()

def mergeTrees(t1, t2):
    """
    :type t1: TreeNode
    :type t2: TreeNode
    :rtype: TreeNode
    """
    if t1 == None and t2 == None:
        return None
    elif t1 == None and t2 != None:
        return t2
    elif t2 == None and t1 != None:
        return t1
    else:
        ans = TreeNode(0)
        ans.val = t1.val+t2.val
        ans.left = mergeTrees(t1.left, t2.left)
        ans.right = mergeTrees(t1.right, t2.right)
        return ans


# sample: 这里根据LeeCode原题的输入建表
null = -1
l1 = [1,3,2,5]     
l2 = [2,1,3,null,4,null,7]
Tree1 = TreeNode(0)
Tree1 = Tree1.CreatTree(l1, 0)
Tree2 = TreeNode(0)
Tree2 = Tree2.CreatTree(l2, 0)

Tree1.PRINT()
print('\n')
Tree2.PRINT()
print('\n')
mergeTrees(Tree1, Tree2).PRINT()
print('\n')