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:**

![](https://assets.leetcode.com/uploads/2020/10/12/ex_depth.jpg)

**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 <code>[0, 10<sup>5</sup>]</code>.
*   `-1000 <= Node.val <= 1000`