Binary Tree Vertical Order Traversal

Posted 2020-08-03 keepshuatishuati

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public List<List<Integer>> verticalOrder(TreeNode root) {
12         List<List<Integer>> result = new ArrayList<>();
13         if (root == null) {
14             return result;
15         }
16         Map<Integer, List<Integer>> levels = new TreeMap<>();
17         Queue<TreeNode> nodeQueue = new LinkedList<>();
18         Queue<Integer> indexQueue = new LinkedList<>();
19         nodeQueue.offer(root);
20         indexQueue.offer(0);
21         while (!nodeQueue.isEmpty()) {
22             int size = nodeQueue.size();
23             for (int i = 0; i < size; i++) {
24                 TreeNode current = nodeQueue.poll();
25                 int index = indexQueue.poll();
26                 
27                 if (current.left != null) {
28                     nodeQueue.offer(current.left);
29                     indexQueue.offer(index - 1);
30                 }
31                 
32                 if (current.right != null) {
33                     nodeQueue.offer(current.right);
34                     indexQueue.offer(index + 1);
35                 }
36                 
37                 if (!levels.containsKey(index)) {
38                     levels.put(index, new ArrayList<>());
39                 }
40                 levels.get(index).add(current.val);
41             }
42         }
43         
44         for (Integer i : levels.keySet()) {
45             result.add(new ArrayList<>());
46             result.get(result.size() - 1).addAll(levels.get(i));
47         }
48         return result;
49     }
50 }

1. Use TreeMap since it‘s keySet is sorted.