题目一：
求一个二叉树的最小深度。
Given a binary tree, find its minimum depth. The minimum depth is the number of nodes aong the shortest path from the root node down to the neartest leaf node.
TreeNode类：
public class TreeNode<T> { public T val; public TreeNode<T> left = null; public TreeNode<T> right = null; public TreeNode<T> parent = null; public TreeNode(T val) { this.val = val; } }