Binary Tree Level Order Traversal (Manna)

public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        List<Integer> level = new ArrayList<Integer>();
        List<TreeNode> queue = new ArrayList<TreeNode>();
        if (root == null)
            return res;
        queue.add(root);
        int len = queue.size();
        TreeNode p;
        while (len!=0) {
        	while (len!=0) {
        		len--;
        		p = queue.get(0);
        		queue.remove(0);
        		level.add(p.val);
        		if (p.left!=null) queue.add(p.left);
        		if (p.right!=null) queue.add(p.right);
        	}
        	len = queue.size();
        	res.add(new ArrayList<Integer>(level));
        	level.clear();	
        }
        return res;
    }
}