import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @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.add(root);

        int depth = 0;
        //使用层序遍历
        while (!queue.isEmpty()){
            int size = queue.size();
            //此时queue 的容量即代表本层有多少节点
            //把下层节点全部push 之后,总深度+1
            for (int i = 0; i < size; i++) {
                TreeNode poll = queue.poll();
                if (poll.left!=null){
                    queue.offer(poll.left);
                }
                if (poll.right!=null){
                    queue.offer(poll.right);
                }
            }
            depth++;
        }
        return depth;
    }
}