Vertical Traversal - Tree - Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> verticalOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if(root == null)
            return result;
        
        Map<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>();
        int min = 0;
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        Queue<Integer> index = new LinkedList<Integer>();
        
        q.add(root);
        index.add(min);
        
        while(!q.isEmpty()) {
            TreeNode current = q.remove();
            int idx = index.remove();
            
            if(!map.containsKey(idx))
                map.put(idx, new ArrayList<Integer>());
            
            map.get(idx).add(current.val);
            
            if(current.left != null) {
                q.add(current.left);
                index.add(idx - 1);
            }
            
            if(current.right != null) {
                q.add(current.right);
                index.add(idx + 1);
            }
            
            min = Math.min(min, idx);
        }
        
        while(map.containsKey(min)) {
            result.add(map.get(min));
            min++;
        }
        
        return result;
    }
}