LeetCode55|二叉树的层次遍历II

1,问题简述

给定一个二叉树，返回其节点值自底向上的层次遍历。

（即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历）

2，示例

例如：给定二叉树 [3,9,20,null,null,15,7],
    3   / \  9  20    /  \   15   7返回其自底向上的层次遍历为：
[  [15,7],  [9,20],  [3]]

3，题解思路

队列的使用

4，题解程序

 import java.util.ArrayList;import java.util.LinkedList;import java.util.List;import java.util.Queue;
public class LevelOrderBottomTest {    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;        t3.right = t5;        List<List<Integer>> listList = levelOrderBottom(t1);        System.out.println("listList = " + listList);    }
    public static List<List<Integer>> levelOrderBottom(TreeNode root) {        List<List<Integer>> listList = new ArrayList<>();        if (root == null) {            return listList;        }        Queue<TreeNode> queue = new LinkedList<>();        queue.add(root);        while (!queue.isEmpty()) {            int size = queue.size();            List<Integer> list = new ArrayList<>();            for (int i = 0; i < size; i++) {                TreeNode node = queue.poll();                list.add(node.val);                if (node.left != null) {                    queue.add(node.left);                }                if (node.right != null) {                    queue.add(node.right);                }            }            listList.add(list);        }        List<List<Integer>> newArrayList = new ArrayList<>(listList.size());        for (int i = listList.size() - 1; i >= 0; i--) {            newArrayList.add(listList.get(i));        }        return newArrayList;    }}