leetcode617 Merge Two Binary Trees

 """
 Given two binary trees and imagine that when you put one of them to cover the other, some nodes of the two trees are overlapped while the others are not.
 You need to merge them into a new binary tree. The merge rule is that if two nodes overlap, then sum node values up as the new value of the merged node. Otherwise, the NOT null node will be used as the node of new tree.
 Example 1:
 Input:
     Tree 1                     Tree 2
           1                         2
          / \                       / \
         3   2                     1   3
        /                           \   \
       5                             4   7
 Output:
 Merged tree:
          3
         / \
        4   5
       / \   \
      5   4   7
 """
 class TreeNode:
     def __init__(self, x):
         self.val = x
         self.left = None
         self.right = None

 class Solution:
     def mergeTrees(self, t1: TreeNode, t2: TreeNode) -> TreeNode:
         if t1 == None:
             return t2
         if t2 == None:
             return t1
         if t1 and t2 == None:
             return None
         result = TreeNode(t1.val + t2.val)
         result.left = self.mergeTrees(t1.left, t2.left)
         result.right = self.mergeTrees(t1.right, t2.right)
         return result
         # Solution2
         # t1.val += t2.val
         # t1.left = self.mergeTrees(t1.left, t2.left)
         # t1.right = self.mergeTrees(t1.right, t2.right)
         # return t1
 """
 此题尚未实现迭代法，但可以省去result空间
 """