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.

Example 1:

    Input: root = [3,9,20,null,null,15,7]
    Output: 2

Example 2:

    Input: root = [2,null,3,null,4,null,5,null,6]
    Output: 5

Constraints:

- The number of nodes in the tree is in the range `[0, 105]`.
- `-1000 <= Node.val <= 1000`