Merge binary tree

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#
#  The class name in the code 、 Method name 、 The parameter name has been specified , Do not modify , Return the value specified by the method directly 
#
# 
# @param t1 TreeNode class  
# @param t2 TreeNode class  
# @return TreeNode class 
#
class Solution:
    def mergeTrees(self , t1: TreeNode, t2: TreeNode) -> TreeNode:
        # write code here
        #  Recursive base item ： For the current two corresponding nodes ：
        # 1.  If there is only one, return the other 
        # 2.  If both exist, then sum 
        # 3.  Then the left subtree and the right subtree of the node are recursive 
        if not t1:
            return t2
        if not t2:
            return t1
        t = TreeNode(t1.val + t2.val)
        t.left = self.mergeTrees(t1.left, t2.left)
        t.right = self.mergeTrees(t1.right, t2.right)
        
        return t