题目描述

不知道这个比其它的解慢多少，求指点

JAVA 代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    class NodeHolder{
        TreeNode node;
        int layer;

        public NodeHolder(TreeNode node, int layer) {
            this.node = node;
            this.layer = layer;
        }

        @Override
        public String toString() {
            return "NodeHolder{" +
                    "node=" + node +
                    ", layer=" + layer +
                    '}';
        }
    }
    public List<List<Integer>> printFromTopToBottom(TreeNode root) {
        if (root == null){
            return new ArrayList<>();
        }
        ArrayList<NodeHolder> list = new ArrayList<>();
        Queue<NodeHolder> queue = new LinkedList<>();
        queue.add(new NodeHolder(root,0));
        while (!queue.isEmpty()){
            NodeHolder holder = queue.poll();
            list.add(holder);
            if (holder.node.left != null){
                queue.add(new NodeHolder(holder.node.left,holder.layer + 1));
            }
            if (holder.node.right != null){
                queue.add(new NodeHolder(holder.node.right,holder.layer + 1));
            }
        }
        // list的最后一个元素的layer属性就是高度
        List<List<Integer>> res = new ArrayList<>();
        for (int i = 0; i <= list.get(list.size() - 1).layer; i ++){
            List<Integer> temp = new ArrayList<>();
            for (NodeHolder holder : list) {
                if (holder.layer == i){
                    temp.add(holder.node.val);
                }
            }
            res.add(temp);
        }
        return res;
    }
}