Given the root of a binary tree, return the leftmost value in the last row of the tree.
Example 1:
Input: root = [2,1,3] Output: 1
Example 2:
Input: root = [1,2,3,4,null,5,6,null,null,7] Output: 7
Constraints:
- The number of nodes in the tree is in the range
[1, 104]. -231 <= Node.val <= 231 - 1