LeetCode | 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.

/**
 * 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;
    }
}