题解 | #从下到上打印二叉树# (Java优质题解)

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型二维数组
     */
    public int[][] levelOrderBottom (TreeNode root) {
        // write code here
        LinkedList<List<Integer>> stack = new LinkedList<>();
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            int l = queue.size();
            List<Integer> t = new ArrayList<>();
            for (int i = 0; i < l; i++) {
                TreeNode node = queue.poll();
                t.add(node.val);
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            stack.add(t);
        }
        List<int[]> res = new ArrayList<>();
        while (!stack.isEmpty()) {
            List<Integer> list = stack.pollLast();
            res.add(list.stream().mapToInt(Integer::intValue).toArray());
        }
        return res.toArray(new int[res.size()][]);

    }
}