给定一个二叉树，返回其节点值自底向上的层序遍历。 （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）

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;

class Solution {

    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        if (root == null) {
            return new ArrayList<>(0);
        }

        List<List<Integer>> ret = new ArrayList<>();

        int deep = 0;
        TreeNode curEnd = root, nextEnd = null;

        LinkedList<TreeNode> queue = new LinkedList<>();

        queue.offer(root);

        while (!queue.isEmpty()) {
            TreeNode cur = queue.poll();

            List<Integer> item;
            if (ret.size() - 1 == deep) {
                item = ret.get(deep);
            } else {
                item = new ArrayList<>();
                ret.add(item);
            }

            item.add(cur.val);

            if (cur.left != null) {
                queue.offer(cur.left);
                nextEnd = cur.left;
            }

            if (cur.right != null) {
                queue.offer(cur.right);
                nextEnd = cur.right;
            }

            if (cur == curEnd) {
                deep++;
                curEnd = nextEnd;
                nextEnd = null;
            }
        }


        Collections.reverse(ret);

        return ret;
    }
}


class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}