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.