[LeetCode]Binary Tree Vertical Order Traversal

public class Solution {
    public List<List<Integer>> verticalOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if (root == null) {
            return result;
        }
        HashMap<TreeNode, Integer> map1 = new HashMap<TreeNode, Integer>();
        TreeMap<Integer, List<Integer>> map2 = new TreeMap<Integer, List<Integer>>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        map1.put(root, 0);
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                int index = map1.get(node);
                List<Integer> list = map2.containsKey(index) ? map2.get(index) : new ArrayList<Integer>();
                list.add(node.val);
                map2.put(index, list);
                if (node.left != null) {
                    queue.offer(node.left);
                    map1.put(node.left, index - 1);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                    map1.put(node.right, index + 1);
                }
            }
        }
        Iterator<Integer> it = map2.keySet().iterator();
        while (it.hasNext()) {
            result.add(map2.get(it.next()));
        }
        return result;
    }
}