# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def mergeTrees(self, t1, t2):
        """
        :type t1: TreeNode
        :type t2: TreeNode
        :rtype: TreeNode
        """
        if not t1 and not t2:
            return None
        t = TreeNode((t1.val if t1 else 0)+(t2.val if t2 else 0))
        if t.left:
            t.left = self.mergeTrees(t1.left, t2.left)
        if t.right:
            t.right = self.mergeTrees(t1.right, t2.right)
        return t
 

更優寫法：

• t1 and t1.left

def mergeTrees(self, t1, t2):
    if not t1 and not t2: return None
    ans = TreeNode((t1.val if t1 else 0) + (t2.val if t2 else 0))
    ans.left = self.mergeTrees(t1 and t1.left, t2 and t2.left)
    ans.right = self.mergeTrees(t1 and t1.right, t2 and t2.right)
    return ans
 

最優解法：

class Solution:
    def mergeTrees(self, t1, t2):
        """
        :type t1: TreeNode
        :type t2: TreeNode
        :rtype: TreeNode
        """
        if not t1:
            return t2
        if not t2:
            return t1
        t1.val += t2.val
        t1.left = self.mergeTrees(t1.left,t2.left)
        t1.right = self.mergeTrees(t1.right,t2.right)
        return t1