Given a binary tree, find its minimum depth.
The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.
Note: A leaf is a node with no children.
Input: root = [3,9,20,null,null,15,7] Output: 2
Input: root = [2,null,3,null,4,null,5,null,6] Output: 5
-1000 <= Node.val <= 1000