[LC] 314. Binary Tree Vertical Order Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int min = Integer.MAX_VALUE;
    int max = Integer.MIN_VALUE;
    public List<List<Integer>> verticalOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        dfs(root, 0);
        for (int i = min; i <= max; i++) {
            res.add(new ArrayList<>());
        }
        
        Queue<TreeNode> queue = new LinkedList<>();
        Queue<Integer> indexQ = new LinkedList<>();
        queue.offer(root);
        // map the leftmost to 0, so that root is -min
        indexQ.offer(-min);
        while (!queue.isEmpty()) {
            TreeNode cur = queue.poll();
            int curIndex = indexQ.poll();
            res.get(curIndex).add(cur.val);
            if (cur.left != null) {
                queue.offer(cur.left);
                indexQ.offer(curIndex - 1);
            }
            if (cur.right != null) {
                queue.offer(cur.right);
                indexQ.offer(curIndex + 1);
            }
        }
        return res;
    }
    
    private void dfs(TreeNode root, int num) {
        if (root == null) {
            return;
        }
        min = Math.min(min, num);
        max = Math.max(max, num);
        dfs(root.left, num - 1);
        dfs(root.right, num + 1);
    }
    
}