<p>You are given two binary trees <code>root1</code> and <code>root2</code>.</p>

<p>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.</p>

<p>Return <em>the merged tree</em>.</p>

<p><strong>Note:</strong> The merging process must start from the root nodes of both trees.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/05/merge.jpg" style="width: 600px; height: 163px;" />
<pre>
<strong>Input:</strong> root1 = [1,3,2,5], root2 = [2,1,3,null,4,null,7]
<strong>Output:</strong> [3,4,5,5,4,null,7]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> root1 = [1], root2 = [1,2]
<strong>Output:</strong> [2,2]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in both trees is in the range <code>[0, 2000]</code>.</li>
	<li><code>-10<sup>4</sup> &lt;= Node.val &lt;= 10<sup>4</sup></code></li>
</ul>

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

class Solution:
    def mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
        # Base case: if both nodes are None, return None
        if not root1 and not root2:
            return None
        
        # If only one of the nodes is None, return the other node
        if not root1:
            return root2
        if not root2:
            return root1
        
        # Both nodes are present, sum their values and merge their children
        merged_node = TreeNode(root1.val + root2.val)
        
        # Recursively merge left children
        merged_node.left = self.mergeTrees(root1.left, root2.left)
        
        # Recursively merge right children
        merged_node.right = self.mergeTrees(root1.right, root2.right)
        
        return merged_node

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