第11周Merge Two Binary Trees

Merge Two Binary Trees

Leetcode algorithms problem 617：Merge Two Binary Trees

• 問題描述

  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

  先判斷兩棵二叉樹是否爲空；爲空直接返回不爲空的樹，然後新建二叉樹，頂部賦值爲兩個要合併二叉樹的頂部值，接着分別使用遞歸對左子樹和右子樹進行之前的過程。

代碼

/**
 * 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;
    }
};

時間複雜度: O(2^n)
空間複雜度: O(n)

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