Minimum Depth of Binary Tree

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.

思路：

1. 递归求解

2. Level Order Traversal
3. 特别注意，左子树为空，需要计算右子树的最小高度。
4. 特别注意，右子树为空，需要计算左子树的最小高度。