Question：

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 ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
        ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
        traverse(root,1,results);
        Collections.reverse(results);
        return results;
    }

	private void traverse(TreeNode root, int level,
			ArrayList<ArrayList<Integer>> results) {
		if(root==null)
			return;
		if(level>results.size())
			results.add(new ArrayList<Integer>());
		results.get(level-1).add(root.val);
		traverse(root.left,level+1,results);
		traverse(root.right,level+1,results);
	}
}

leetcode JAVA Binary Tree Level Order Traversal II 难度系数3 3.39