[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;
}
}