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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0601_0700.s0617_merge_two_binary_trees;

// #Easy #Depth_First_Search #Breadth_First_Search #Tree #Binary_Tree
// #Algorithm_I_Day_8_Breadth_First_Search_Depth_First_Search
// #2022_03_21_Time_1_ms_(72.47%)_Space_50.8_MB_(48.77%)

import com_github_leetcode.TreeNode;

/*
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
/**
 * 617 - Merge Two Binary Trees\.
 *
 * Easy
 *
 * You are given two binary trees `root1` and `root2`.
 *
 * 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 the two trees 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 the new tree.
 *
 * Return _the merged tree_.
 *
 * **Note:** The merging process must start from the root nodes of both trees.
 *
 * **Example 1:**
 *
 * ![](https://assets.leetcode.com/uploads/2021/02/05/merge.jpg)
 *
 * **Input:** root1 = [1,3,2,5], root2 = [2,1,3,null,4,null,7]
 *
 * **Output:** [3,4,5,5,4,null,7] 
 *
 * **Example 2:**
 *
 * **Input:** root1 = [1], root2 = [1,2]
 *
 * **Output:** [2,2] 
 *
 * **Constraints:**
 *
 * *   The number of nodes in both trees is in the range `[0, 2000]`.
 * *   -10⁴ <= Node.val <= 10⁴
**/
public class Solution {
    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if (root1 == null) {
            return root2;
        }
        if (root2 == null) {
            return root1;
        }
        return new TreeNode(
                root1.val + root2.val,
                mergeTrees(root1.left, root2.left),
                mergeTrees(root1.right, root2.right));
    }
}