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 int maxDepth (TreeNode root) {
        // write code here
        if(root == null) {
            return 0;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int depth = 0;
        while(!queue.isEmpty()) {
            int size = queue.size();
            List<TreeNode> list = new ArrayList<>(size);
            for(int i = 0; i< size; i++) {
                list.add(queue.poll());
            }
            depth++;

            for(int i = 0; i< size; i++) {
                if(list.get(i).left != null) {
                    queue.offer(list.get(i).left);
                }
                if(list.get(i).right != null) {
                    queue.offer(list.get(i).right);
                }                                
            }
        }

        return depth;
    }

}