Print a Binary Tree in Vertical Order

http://www.geeksforgeeks.org/print-binary-tree-vertical-order/

package algorithms;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;

public class PrintVertical {

    private static TreeMap<Integer, List<TreeNode>> hm = new TreeMap<>();
    
    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        TreeNode two = new TreeNode(2);
        TreeNode three = new TreeNode(3);
        TreeNode four = new TreeNode(4);
        TreeNode five = new TreeNode(5);
        TreeNode six = new TreeNode(6);
        TreeNode seven = new TreeNode(7);
        TreeNode eight = new TreeNode(8);
        TreeNode nine = new TreeNode(9);
        root.left = two;
        root.right = three;
        two.left = four;
        two.right = five;
        three.left = six;
        three.right = seven;
        six.right = eight;
        seven.right = nine;
        printVertical(root);
        for(Map.Entry<Integer, List<TreeNode>> entry: hm.entrySet()) {
            System.out.println("key====="+entry.getKey());
            List<TreeNode> temp = entry.getValue();
            for(TreeNode node: temp){
                System.out.println("node val======"+node.val);
            }
        }
    }
    
    public static void printVertical(TreeNode root) {
        helper(0, root);
    }
    
    public static void helper(int cur, TreeNode root) {
        if(root == null)
            return;

        ArrayList<TreeNode> array;
        if(hm.containsKey(cur)) {
            array = (ArrayList<TreeNode>) hm.get(cur);
        } else {
            array = new ArrayList<TreeNode>();
        }
        array.add(root);
        hm.put(cur, array);
        helper(cur - 1, root.left);
        helper(cur + 1, root.right);
    }
}

<注> 需要注意的是使用TreeMap而不是HashMap来存遍历以后的数据。Better to use TreeMap in this case, as it will store the result in ascending order, 

    which we are interested in.