Given two binary trees, t1 and t2, return the merged tree.

To merge the two binary trees:

If two nodes overlap, their values are summed to form the new node value.

If a node is null in one tree, the non-null node from the other tree is used in the merged tree

Note: The merging process must start from the root nodes of both trees.

When overlaying one tree on top of the other, some nodes overlap while others do not.

Example 1:

Input: t1 = [1,4,3,6], t2 = [2,1,4,null,9,null,8]
Output: [3,5,7,6,9,null,8]
Explanation: Example: Node 4 and node 1, both with a yellow background, overlap. The resulting node has a value of 5, which is the sum of 4 and 1.

Example 2:

Input: t1 = [2], t2 = [2,3]
Output: [4,3]
Explanation: The root nodes overlap, so 2 + 2 = 4. The left child 3 exists only in t2, so it is kept in the merged tree.

Example 3:

Input: t1 = [], t2 = [5,1,7]
Output: [5,1,7]
Explanation: Since t1 is empty, the merged tree is exactly t2.

Constraints

The number of nodes in each tree is in the range [0, 2000]

-10⁴<= Node.val <= 10⁴

35. Merge Binary Trees