617. Merge Two Binary Trees

问题描述

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.

思路

recursive
把第二棵树的值，直接加到第一棵树上，或者重新创建一棵树

class Solution(object):
    def mergeTrees(self, t1, t2):
        """
        :type t1: TreeNode
        :type t2: TreeNode
        :rtype: TreeNode
        """
        if not t1 and not t2: return None
        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

class Solution(object):
    def mergeTrees(self, t1, t2):
        """
        :type t1: TreeNode
        :type t2: TreeNode
        :rtype: TreeNode
        """
        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.left, t2.left)
        ans.right = self.mergeTrees(t1.right, t2.right)
        return ans