617. 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:

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题解：

class BTNode:
    def __init__(self, value):
        self.value = value
        self.lchild = None
        self.rchild = None


def merge(root1, root2):
    if not root1 and not root2:
        return None

    if not root1:
        return root2

    if not root2:
        return root1

    root = BTNode(root1.value + root2.value)
    root.lchild = merge(root1.lchild, root2.lchild)
    root.rchild = merge(root1.rchild, root2.rchild)
    return root


def inorder_traversal(root):
    if not root:
        return

    inorder_traversal(root.lchild)
    print(root.value, end=' ')
    inorder_traversal(root.rchild)


if __name__ == '__main__':
    root1 = BTNode(1)
    root1.lchild = BTNode(3)
    root1.rchild = BTNode(2)
    root1.lchild.lchild = BTNode(5)

    root2 = BTNode(2)
    root2.lchild = BTNode(1)
    root2.rchild = BTNode(3)
    root2.lchild.rchild = BTNode(4)
    root2.rchild.rchild = BTNode(7)

    root = merge(root1, root2)
    inorder_traversal(root)
    print()