0314. Binary Tree Vertical Order Traversal

https://leetcode.com/problems/binary-tree-vertical-order-traversal

Description

Given the root of a binary tree, return the vertical order traversal of its nodes' values. (i.e., 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.

Example 1:

**Input:** root = [3,9,20,null,null,15,7]
**Output:** [[9],[3,15],[20],[7]]

Example 2:

**Input:** root = [3,9,8,4,0,1,7]
**Output:** [[4],[9],[3,0,1],[8],[7]]

Example 3:

**Input:** root = [3,9,8,4,0,1,7,null,null,null,2,5]
**Output:** [[4],[9,5],[3,0,1],[8,2],[7]]

Example 4:

**Input:** root = []
**Output:** []

Constraints:

• The number of nodes in the tree is in the range [0, 100].

• -100 <= Node.val <= 100

ac

/**
 * 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>> res = new ArrayList<List<Integer>>();
        // edge case
        if (root == null) return res;

        TreeMap<Integer, List<Integer>> tm = new TreeMap<>();

        // BFS
        Queue<TreeNode> q = new LinkedList<>();
        tm.put(0, new ArrayList<Integer>());
        tm.get(0).add(root.val);
        root.val = 0;
        q.offer(root);

        while (!q.isEmpty()) {
            TreeNode curr = q.poll();

            // left
            if (curr.left != null) {
                if (!tm.containsKey(curr.val-1)) tm.put(curr.val-1, new ArrayList<Integer>());
                tm.get(curr.val-1).add(curr.left.val);
                curr.left.val = curr.val-1;
                q.offer(curr.left);
            }
            // right
            if (curr.right != null) {
                if (!tm.containsKey(curr.val+1)) tm.put(curr.val+1, new ArrayList<Integer>());
                tm.get(curr.val+1).add(curr.right.val);
                curr.right.val = curr.val+1;
                q.offer(curr.right);
            }

        }

        for (int i : tm.keySet()) {
            res.add(tm.get(i));
        }

        return res;
    }
}

 3 /\ / \ 9 8 /\ /\ / \/ \ 4 01 7 /\ / \ 5 2 

 [ [4], [9,5], [3,0,1], [8,2], [7] ]