链接：https://leetcode.com/problems/minimum-depth-of-binary-tree/description/
难度：Easy
题目：111. 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.

翻译：给定一棵二叉树，求它的最小深度。 最小深度是沿着从根节点到最近的叶节点的最短路径的节点的数量。

思路：本题与Maximum Depth of Binary Tree类似，依旧用递归的方法来求解。不过，有个地方要注意一下，左子树或者右子树为空时，我们不能直接让其深度等于0，而要转去求非空的右子树或者左子树的深度。

参考代码：
Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int minDepth(TreeNode root) {
        if(root==null)
            return 0;
        if(root.left == null)
            return 1+minDepth(root.right);
        if(root.right == null)
            return 1+minDepth(root.left);
        return 1+Math.min(minDepth(root.left), minDepth(root.right));
    }
}