Binary Tree Vertical Order Traversal

Binary Tree Vertical Order Traversal hash table

Given a binary tree, return the vertical order traversal of its nodes’ values. (ie, from top to bottom, column by column).

If two nodes are in the same row and column, the order should be from left to right.

Examples:

Given binary tree [3,9,20,null,null,15,7],
    3
   /\
  /  \
  9  20
     /\
    /  \
   15   7
return its vertical order traversal as:
 [
   [9],
   [3,15],
   [20],
   [7]
 ]
Given binary tree [3,9,8,4,0,1,7],
      3
     /\
    /  \
    9   8
   /\  /\
  /  \/  \
  4  01   7
return its vertical order traversal as:
 [
   [4],
   [9],
   [3,0,1],
   [8],
   [7]
 ]
Given binary tree [3,9,8,4,0,1,7,null,null,null,2,5] (0's right child is 2 and 1's left child is 5),
      3
     /\
    /  \
    9   8
   /\  /\
  /  \/  \
  4  01   7
     /\
    /  \
    5   2
return its vertical order traversal as:
 [
   [4],
   [9,5],
   [3,0,1],
   [8,2],
   [7]
 ]

Binary Tree Vertical Order Traversal Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class BinaryTreeVerticalOrderTraversal {
    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;
        int 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;
    }
}

Last modified October 4, 2020