111. Minimum Depth of Binary Tree

题目

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.

题目大意

给定一个二叉树，找出其最小深度。最小深度是从根节点到最近叶子节点的最短路径上的节点数量。说明: 叶子节点是指没有子节点的节点。

解题思路

• 递归求出根节点到叶子节点的深度，输出最小值即可