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 int[][] levelOrderBottom (TreeNode root) {
        // write code here

        
        Stack<int[]> levels = new Stack<>();

        Queue<TreeNode> q = new LinkedList<>();

        q.offer(root);


        while (!q.isEmpty()) {
            int size = q.size();
            int[] level = new int[size];
            int i = 0;
            while(i < size) {
                TreeNode node = q.poll();
                if (node.left != null) {
                    q.offer(node.left);
                }
                if (node.right != null) {
                    q.offer(node.right);
                }
                level[i] = node.val;
                i++;
            }
            levels.push(level);
        }

        int[][] ans = new int[levels.size()][];
        int i = 0;
        while(!levels.isEmpty()) {
            ans[i] = levels.pop();
            i++;
        }

        return ans;


    }
}

1、层次遍历，记录每一行，进栈；

2、出栈，即答案。