verticalOrder.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<>();
        if(root == null) return result;
        
        Map<Integer, ArrayList<Integer>> map = new HashMap<>();
        Queue<TreeNode> q = new LinkedList<>();
        Queue<Integer> cols = new LinkedList<>();
        
        q.add(root);
        cols.add(0);
        
        int min=0, max=0;
        
        while(!q.isEmpty())
        {
            TreeNode node = q.poll();
            int col = cols.poll();
            
            if(!map.containsKey(col))
                map.put(col, new ArrayList<Integer>());
            
            map.get(col).add(node.val);
            
            if(node.left != null)
            {
                q.add(node.left);
                cols.add(col-1);
                min = Math.min(min, col-1);
            }
            
            if(node.right != null)
            {
                q.add(node.right);
                cols.add(col+1);
                max = Math.max(max, col+1);
            }
        }
        
        for(int i=min; i<=max; i++)
            result.add(map.get(i));
        
        return result;
    }
}