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},
Paste_Image.png
代码
/**
* 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<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
// write your code here
if(root == null)
return new ArrayList<>();
ArrayList<ArrayList<Integer>> res = new ArrayList<>();
Queue<TreeNode> list = new LinkedList<>();
list.offer(root);
while(!list.isEmpty()) {
int size = list.size();
ArrayList<Integer> level = new ArrayList<>();
for(int i=0;i<size;i++) {
TreeNode cur = list.poll();
level.add(cur.val);
if(cur.left != null)
list.offer(cur.left);
if(cur.right != null)
list.offer(cur.right);
}
res.add(level);
}
Collections.reverse(res);
return res;
}
}
