Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private TreeMap<Integer, List<int[]>> d = new TreeMap<>();

    public List<List<Integer>> verticalOrder(TreeNode root) {
        dfs(root, 0, 0);
        List<List<Integer>> ans = new ArrayList<>();
        for (var v : d.values()) {
            Collections.sort(v, (a, b) -> a[0] - b[0]);
            List<Integer> t = new ArrayList<>();
            for (var e : v) {
                t.add(e[1]);
            }
            ans.add(t);
        }
        return ans;
    }

    private void dfs(TreeNode root, int depth, int offset) {
        if (root == null) {
            return;
        }
        d.computeIfAbsent(offset, k -> new ArrayList<>()).add(new int[] {depth, root.val});
        dfs(root.left, depth + 1, offset - 1);
        dfs(root.right, depth + 1, offset + 1);
    }
}