二叉树遍历（由子节点往父节点遍历）

 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class  {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
       
       List<List<Integer>> wrapList=new LinkedList<List<Integer>>();
         if(root==null)
            return wrapList;
        Queue<TreeNode> queue=new LinkedList<TreeNode>();
        queue.offer(root);
        while(!queue.isEmpty())
        {
            int levelNum=queue.size();
            List<Integer> subList=new LinkedList<Integer>();
            for(int i=0;i<levelNum;i++)
            {
                if(queue.peek().left!=null) 
                    queue.offer(queue.peek().left);
                if(queue.peek().right!=null) 
                    queue.offer(queue.peek().right);
                subList.add(queue.poll().val);
            }
            wrapList.add(0,subList);
        }
            return wrapList;
    }
}