public class Solution
extends Object
111 - Minimum Depth of Binary Tree\.
Easy
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`