（难度Easy） Problem 111. Minimum Depth of Binary Tree（二叉树最浅深度）

Minimum Depth of Binary Tree

Easy

## Description

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:

Given binary tree [3,9,20,null,null,15,7],

    3
/ \
9  20
/  \
15   7

return its minimum depth = 2.