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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

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

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> reList = new ArrayList<List<Integer>>();
        List<List<TreeNode>> nodeList = new ArrayList<List<TreeNode>>();
        List<TreeNode> treList = new ArrayList<TreeNode>();
        if(root==null) return reList;
        int i = 0;
        treList.add(root);
        nodeList.add(treList);
        boolean flag = true;
        while(flag) {
            flag = false;
            treList = nodeList.get(i++);
            List<TreeNode> tempList = new ArrayList<TreeNode>();
            for(TreeNode node:treList) {
               if(node.left!=null) {
                   tempList.add(node.left);
                   flag = true;
               }
               if(node.right!=null) {
                   tempList.add(node.right);
                   flag = true;
               }
            }
            if(!tempList.isEmpty()) nodeList.add(tempList);
        }
        for(int j = nodeList.size()-1;j>=0;j--) {
            treList = nodeList.get(j);
            List<Integer> tempList = new ArrayList<Integer>();
            for(TreeNode node:treList) {
                tempList.add(node.val);
            }
            reList.add(tempList);
        }
        return reList;
        
    }
}