第11周Merge Two Binary Trees

Merge Two Binary Trees

Leetcode algorithms problem 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.

• 例子

  Input:
  Tree 1 Tree 2
  1 2
  / \ / \
  3 2 1 3
  / \ \
  5 4 7
  Output:
  Merged tree:
  3
  / \
  4 5
  / \ \
  5 4 7

• 思路

  先判断两棵二叉树是否为空；为空直接返回不为空的树，然后新建二叉树，顶部赋值为两个要合并二叉树的顶部值，接着分别使用递归对左子树和右子树进行之前的过程。

代码

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* mergeTrees(TreeNode* t1, TreeNode* t2) {
        if (!t1) return t2;
        if (!t2) return t1;

        TreeNode* result = new TreeNode(t1->val + t2->val);
        result->left = mergeTrees(t1->left, t2->left);
        result->right = mergeTrees(t1->right, t2->right);
        return result;
    }
};

时间复杂度: O(2^n)
空间复杂度: O(n)

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