二叉树的深度

输入一棵二叉树，求该树的深度。从根结点到叶结点依次经过的结点（含根、叶结点）形成树的一条路径，最长路径的长度为树的深度

解题思路

递归写法，比较简单

public class Solution {
    
    public int TreeDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }
        int left = TreeDepth(root.left);
        int right = TreeDepth(root.right);
        return Math.max(left, right) + 1;
    }
}

非递归写法

depth 是当前结点所在的层数，count 是已经遍历过的结点数，nextCount 是下层的结点总数，当 count == nextCount 的时候，代表本层的节点已经遍历完毕

import java.util.LinkedList;
import java.util.Queue;
public class Solution {
    
    public int TreeDepth(TreeNode root) {
        if(root == null) {
            return 0;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int depth = 0, count = 0, nextCount = 1;
        while(queue.size() != 0) {
            TreeNode top = queue.poll();
            count++;
            if(top.left != null) {
                queue.add(top.left);
            }
            if(top.right != null) {
                queue.add(top.right);
            }
            if(count == nextCount) {
                nextCount = queue.size();
                count = 0;
                depth++;
            }
        }
        return depth;
    }
}