题解 | #从下到上打印二叉树#

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 ArrayList<ArrayList<TreeNode>> sol(TreeNode root){
        ArrayList<ArrayList<TreeNode>> res = new ArrayList<ArrayList<TreeNode>>();
        if(root == null) return res;
        Deque<TreeNode> deq = new ArrayDeque<TreeNode>();
        deq.add(root);
        while(!deq.isEmpty()){
            int len = deq.size();
            ArrayList<TreeNode> temp = new ArrayList<TreeNode>();
            for(int i = 0; i < len; i++){
                TreeNode node = deq.remove();
                temp.add(node);
                if(node.left!=null)deq.add(node.left);
                if(node.right!=null)deq.add(node.right);
            }
            res.add(temp);
        }
        return res;
    }


    public int[][] levelOrderBottom (TreeNode root) {
        // write code here
        ArrayList<ArrayList<TreeNode>> res = sol(root);
        Collections.reverse(res);
        int[][] ans = new int[res.size()][];
        for(int i = 0; i < res.size();i++){
            ArrayList<TreeNode> temp = res.get(i);
            int[] arr = new int[temp.size()];
            for(int j = 0; j < temp.size();j++){
                arr[j] = temp.get(j).val;
            }
            ans[i] = arr;
        }
        return ans;
    }
}