leetcode题记——Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> anw = new ArrayList<List<Integer>>();
        List<Integer> floor = new ArrayList<Integer>();
        List<TreeNode> treeNodeList = new ArrayList<TreeNode>();
        if(root == null){
            return anw;
        }
        floor.add(root.val);
        anw.add(floor);
        treeNodeList.add(root);
        while (true) {
            List<Integer> value = new ArrayList<Integer>();
            List<TreeNode> tree = new ArrayList<TreeNode>();
            for (int i = 0; i < treeNodeList.size(); i++) {
                if(treeNodeList.get(i).left != null){
                    tree.add(treeNodeList.get(i).left);
                    value.add(treeNodeList.get(i).left.val);
                }
                if(treeNodeList.get(i).right != null){
                    tree.add(treeNodeList.get(i).right);
                    value.add(treeNodeList.get(i).right.val);
                }
            }
            if(tree.size() == 0){
                break;
            }else{
                treeNodeList = tree;
                anw.add(value);
            }
        }
        Collections.reverse(anw);
        return anw;
    }
}