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.

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */

//注意题目给出的二叉树深度的概念：根节点到“叶子”节点的路径长。//对{root!=null, left=right=null}的单节点树，其深度为1 [注：也有教材把单节点树的深度定义为0，leetcode定义为1，否则不accepted]//在求最小深度时，要避免那些只有单个子节点的节点的干扰，因为其为null的子节点会返回0，在取小比较时可能造成其为叶子节点的假象//而在求最大深度时，单个子节点为空的非叶子节点不会干扰到结果，因为其返回的0在比较大小取大时会被淘汰，从而间接证明其非叶子节点public class Solution {    public int minDepth(TreeNode root) {        if (root == null) return 0;                int minleft = minDepth(root.left);        int minright = minDepth(root.right);                if(minleft == 0) return minright+1;  //左子节点为null，则其不是叶子节点，只是深度路径上的中间点而已        if(minright == 0) return minleft+1;                int depth = minleft<=minright?minleft:minright;        return depth+1;    }}