Problem URL: Merge two binary trees

We will start form the root and run a DFS with both the tree and merge their value to the new tree and return that new tree as result.

# 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]:
        if root1 and root2:
            root = TreeNode(root1.val + root2.val)
            root.left = self.mergeTrees(root1.left, root2.left)
            root.right = self.mergeTrees(root1.right, root2.right)
            return root
        else:
            return root1 or root2

Time Complexity: O(n)
Space Complexity: O(1)