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.

```
Input: root1 = [1,3,2,5], root2 = [2,1,3,null,4,null,7]
Output: [3,4,5,5,4,null,7]
```

```
Input: root1 = [1], root2 = [1,2]
Output: [2,2]
```

```
class Solution:
def mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
def dfs(ptr1,ptr2):
ptr1.val+=ptr2.val
if ptr1.left and ptr2.left: dfs(ptr1.left,ptr2.left)
elif ptr2.left:
ptr1.left=ptr2.left
if ptr1.right and ptr2.right: dfs(ptr1.right,ptr2.right)
elif ptr2.right:
ptr1.right=ptr2.right
start1,start2=root1,root2
if start1 and start2:
dfs(start1,start2)
elif start1:
return start1
else:
return start2
return root1
```