Minimum Depth of Binary Tree

题目地址：
https://leetcode.com/problems/minimum-depth-of-binary-tree/#/description

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.

解题思路：
这道题主要就是从左边要一个值，从右边要一个值，然后比较大小。

代码：

public int minDepth(TreeNode root) {
    if(root == null){
        return 0;
    }
    if(root.left == null){
        return 1 + minDepth(root.right);
    }
    else if(root.right == null){
        return 1 + minDepth(root.left);
    }
    return 1 + Math.min(minDepth(root.left), minDepth(root.right));
}