617. Merge Two Binary Trees; Difficulty:Easy-优快云博客

本文介绍了一种将两棵二叉树合并成一棵新二叉树的算法，合并规则为若有节点重叠，则将节点值相加作为新节点值；若无重叠则保留非空节点。文章提供了C语言及Python实现代码。

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

Note: The merging process must start from the root nodes of both trees.

算法分析：运用递归，两棵树同时先序遍历。（1）当t1为空，t2不空的时候，就把t2节点放回到t1的父结点；（2）当t1不空，t2空，那么原封不动，直接返回t1结点；（3）当t1和t2都不为空时，把t2的值加到t1，然后返回t1。最后返回的时t1树的根节点。

C语言版

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
struct TreeNode* mergeTrees(struct TreeNode* t1, struct TreeNode* t2) {
    if(t1 && t2)
    {
        t1->val += t2->val;
        t1->left = mergeTrees(t1->left, t2->left);
        t1->right = mergeTrees(t1->right, t2->right);
    }
    else if(t2)
        return t2;

    return t1;
}

Python版

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

class Solution(object):
    def mergeTrees(self, t1, t2):
        """
        :type t1: TreeNode
        :type t2: TreeNode
        :rtype: TreeNode
        """
        if(t1 and t2):
            t1.val += t2.val
            t1.left = self.mergeTrees(t1.left, t2.left)
            t1.right = self.mergeTrees(t1.right, t2.right)
        elif(t2):
            return t2;
        return t1