一、题目

　　从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。
二、思路

       队列LinkedList完成层序遍历，用end记录每层结点数目

三、代码

import java.util.ArrayList;
import java.util.LinkedList;

/*
* 队列LinkedList完成层序遍历，用end记录每层结点数目
*/
public class Solution {
    ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        //保存结果
        ArrayList<ArrayList<Integer>> result = new ArrayList<>();
        //预检验
        if (pRoot == null) {
            return result;
        }
        LinkedList<TreeNode> layer = new LinkedList<>();
        ArrayList<Integer> layerList = new ArrayList<>();

        layer.add(pRoot);

        int start = 0, end = 1;

        while (!layer.isEmpty()) {
            TreeNode cur = layer.remove();
            layerList.add(cur.val);
            start++;
            if (cur.left != null) {
                layer.add(cur.left);
            }
            if (cur.right != null) {
                layer.add(cur.right);
            }
            if (start == end) {
                end = layer.size();
                start = 0;
                result.add(layerList);
                layerList = new ArrayList<>();
            }
        }
        return result;
    }
}

View Code

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参考链接：

https://www.nowcoder.com/practice/445c44d982d04483b04a54f298796288?tpId=13&tqId=11213&rp=3&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking