LeetCode22|从上到下打印二叉树II

码农王同学

发布于 2020-08-25 11:22:13

1960

发布于 2020-08-25 11:22:13

文章被收录于专栏：后端Coder

1，问题简述

从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。

2，示例

给定二叉树: [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
返回其层次遍历结果：

[
  [3],
  [9,20],
  [15,7]
]

3，题解思路

队列的使用，这样的题上面已经说过一次了，这里就看下这里的题解程序吧

4，题解程序


import java.util.*;

public class LevelOrderTest2 {
    public static void main(String[] args) {
        TreeNode t1 = new TreeNode(3);
        TreeNode t2 = new TreeNode(9);
        TreeNode t3 = new TreeNode(20);
        TreeNode t4 = new TreeNode(15);
        TreeNode t5 = new TreeNode(7);
        t1.left = t2;
        t1.right = t3;
        t3.left = t4;
        t4.right = t5;
        List<List<Integer>> listList = levelOrder(t1);
        System.out.println("listList = " + listList);
    }

    public static List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> list = new ArrayList<>();
        if (root == null) {
            return list;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            List<Integer> newList = new ArrayList<>();
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                newList.add(node.val);
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            list.add(newList);
        }
        return list;
    }


}