题目

给出一棵二叉树，返回其节点值从底向上的层次序遍历（按从叶节点所在层到根节点所在的层遍历，然后逐层从左往右遍历）

样例

给出一棵二叉树 {3,9,20,#,#,15,7},

3   / \  9  20    /  \   15   7

按照从下往上的层次遍历为：

[  [15,7],  [9,20],  [3]]

解题

和的答案反过来

直接每次在list中第一个位置加入当前层结点

/** * Definition of TreeNode: * public class TreeNode { *     public int val; *     public TreeNode left, right; *     public TreeNode(int val) { *         this.val = val; *         this.left = this.right = null; *     } * } */  public class Solution {    /**     * @param root: The root of binary tree.     * @return: buttom-up level order a list of lists of integer     */    public ArrayList
      
       
        > levelOrderBottom(TreeNode root) {        // write your code here        ArrayList
        
         
          > tree = new ArrayList
          
           
            >(); if(root == null) return tree; Queue
            
              queue = new LinkedList
             
              (); queue.offer(root); while(!queue.isEmpty()){ ArrayList
              
                list = new ArrayList
               
                (); int size = queue.size(); for(int i = 0;i