# 617. Merge Two Binary Trees

### Description

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.

### Constraints

* The number of nodes in both trees is in the range `[0, 2000]`.
* `-104 <= Node.val <= 104`

### Approach

### Links

* GeeksforGeeks
* [Leetcode](https://leetcode.com/problems/merge-two-binary-trees/)
* ProgramCreek
* YouTube

### **Examples**

{% tabs %}
{% tab title="Example 1" %}
**Input:** root1 = \[1, 3, 2, 5], root2 = \[2, 1, 3, null, 4, null, 7]

**Output:** \[3, 4, 5, 5, 4, null, 7]

![](/files/-MUCcvO9TP_RA9FYdABm)
{% endtab %}

{% tab title="Example 2" %}
**Input:** root1 = \[1], root2 = \[1, 2]

**Output:** \[2, 2]
{% endtab %}
{% endtabs %}

### **Solutions**

{% tabs %}
{% tab title="TreeNode" %}

```java
// 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;
		}
}
```

{% endtab %}

{% tab title="Solution 1" %}

```java
/**
 * Time complexity : O(m). A total of m nodes need to be traversed. 
 *    Here, m represents the minimum number of nodes from the two given trees.
 * Space complexity : O(m). The depth of the recursion tree can go upto m 
 *    in the case of a skewed tree. In average case, depth will be O(logm).
 */

class Solution {
    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if(root1 == null && root2 == null) {
            return null;
        }
        if(root1 == null) {
            return root2;
        }
        if(root2 == null) {
            return root1;
        }
        root1.val += root2.val;
        root1.left = mergeTrees(root1.left, root2.left);
        root1.right = mergeTrees(root1.right, root2.right);
        
        return root1;
    }
}
```

{% endtab %}

{% tab title="Solution 2" %}

```java
/**
 * Time complexity : O(n). We traverse over a total of n nodes. 
 *    Here, n refers to the smaller of the number of nodes in the two trees.
 * Space complexity : O(n). The depth of stack can grow upto n in case of a skewed tree.
 */

class Solution {
    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if(root1 == null && root2 == null) {
            return null;
        }
        if(root1 == null) {
            return root2;
        }
        if(root2 == null) {
            return root1;
        }
        Stack<TreeNode[]> stack = new Stack();
        stack.push(new TreeNode[]{root1, root2});
        
        while(!stack.isEmpty()) {
            TreeNode[] t = stack.pop();
            if(t[1] == null) {
                continue;
            }
            t[0].val += t[1].val;
            if(t[0].left == null) {
                t[0].left = t[1].left;
            } else {
                stack.push(new TreeNode[]{t[0].left, t[1].left});
            }
            if(t[0].right == null) {
                t[0].right = t[1].right;
            } else {
                stack.push(new TreeNode[]{t[0].right, t[1].right});
            }
        }
        
        return root1;
    }
}
```

{% endtab %}
{% endtabs %}

### **Follow up**

*


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://code-snippets.hbamithkumara.com/leetcode/problems/601-700/merge-two-binary-trees.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.