题解 | #二叉树的最大深度#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return int整型
     */

    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here

        ArrayList<ArrayList<Integer>> list = new ArrayList();
        if (root == null) return list;
        Queue<TreeNode> q = new ArrayDeque<TreeNode>();
        q.add(root);
        while (!q.isEmpty()) {
            ArrayList<Integer> temp = new ArrayList<Integer>();
            int n = q.size();
            for (int i = 0; i < n; i++) {
                TreeNode cur = q.poll();
                temp.add(cur.val);
                if (cur.left != null) q.add(cur.left);
                if (cur.right != null) q.add(cur.right);
            }
            list.add(temp);

        }


        return list;
    }
    public int maxDepth (TreeNode root) {
        // write code here
        ArrayList<ArrayList<Integer>> list;
        if(root==null) return 0;

        list = levelOrder(root);
        int n = list.size();

        return n;
    }
}

在做层序遍历时注意到，大的ArrayList中的小ArrayList的个数，就是该二叉树的最大深度。