合并二叉树

image.png

递归解法：

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def mergeTrees(self, root1, root2):
        """
        :type root1: TreeNode
        :type root2: TreeNode
        :rtype: TreeNode
        """
        root = TreeNode()
        
        def merge(node1, node2):
            # node = TreeNode()
            if not node1 and not node2:
                return None
            if node1 and node2:
                node = TreeNode(node1.val + node2.val)
                node.left = merge(node1.left, node2.left)
                node.right = merge(node1.right, node2.right)
            elif not node1:
                node = node2
            elif not node2:
                node = node1
            return node
        
        return merge(root1,root2)

image.png

也可以改成只修改其中一棵树的节点值，而非创建一颗新的二叉树，以节省一点空间。

class Solution(object):
    def mergeTrees(self, root1, root2):
        """
        :type root1: TreeNode
        :type root2: TreeNode
        :rtype: TreeNode
        """
        root = TreeNode()
        
        def merge(node1, node2):
            # node = TreeNode()
            if not node1 and not node2:
                return None
            if node1 and node2:
                node1.val = node1.val + node2.val
                node1.left = merge(node1.left, node2.left)
                node1.right = merge(node1.right, node2.right)
                return node1
            elif not node1:
                return node2
            elif not node2:
                return node1

        return merge(root1,root2)

image.png