题目描述：
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.
题目大意：
给定一棵二叉树，计算其最小深度。
最小深度是指从根节点出发到达最近的叶子节点所需要经过的节点个数。
解题思路：
DFS或者BFS均可，详见代码。
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.
给定一棵二叉树，计算其最小深度。
最小深度是指从根节点出发到达最近的叶子节点所需要经过的节点个数。
DFS或者BFS均可，详见代码。