java 代码

//import java.util.*;
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

class Solution {
    public List<List<Integer>> printFromTopToBottom(TreeNode root) {
        List<List<Integer>> res=new ArrayList<>();
        if(root==null) return res;
        Queue<TreeNode> stack=new LinkedList<>();
        stack.offer(root);
        boolean T= true;
        int cnt=1,tmp=0;
        List<Integer> list=new ArrayList<>();
        while(!stack.isEmpty()){
            TreeNode cur=stack.poll();
            if(T) list.add(cur.val);
            else list.add(0,cur.val);
            cnt--;
            if(cur.left!=null) {
                stack.offer(cur.left);
                tmp++;
            }
            if(cur.right!=null) {
                stack.offer(cur.right);
                tmp++;
            }
            if(cnt==0){
                cnt=tmp;
                tmp=0;
                res.add(list);
                list=new ArrayList<>();
                T=!T;
            }
        }
        return res;
    }
}