314. Binary Tree Vertical Order Traversal

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Example 1:

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

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Example 2:

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

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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]]

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Example 4:

Input: root = []
Output: []

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/**
 * 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 {
    public List<List<Integer>> verticalOrder(TreeNode root) {
        List<List<Pair<Integer, Integer>>> posList = new ArrayList<>();
        List<List<Pair<Integer, Integer>>> negList = new ArrayList<>();
        dfs(root, 0, 0, negList, posList);
        
        return merge(negList, posList);
    }
    
    void dfs(TreeNode node, int row, int col, 
             List<List<Pair<Integer, Integer>>> negList,
             List<List<Pair<Integer, Integer>>> posList
            ) {
        
        if (node == null) return;
        
        if (col >= 0) {
            if (posList.size() == col) {
                posList.add(new ArrayList<>());
            }
            posList.get(col).add(new Pair<>(node.val, row));
        } else {
            int posCol = -col - 1;
            if (negList.size() == posCol) {
                negList.add(new ArrayList<>());
            }
            negList.get(posCol).add(new Pair<>(node.val, row));
        }
        
        dfs(node.left, row + 1, col - 1, negList, posList);
        dfs(node.right, row + 1, col + 1, negList, posList);
    }
    
    List<List<Integer>> merge(List<List<Pair<Integer, Integer>>> negList, 
                              List<List<Pair<Integer, Integer>>> posList) {
        
        List<List<Integer>> res = new ArrayList<>();
        
        for (int i = negList.size() - 1; i >= 0; i--) {
            negList.get(i).sort((a, b) -> a.getValue() - b.getValue());
            res.add(negList.get(i).stream().map(a -> a.getKey()).collect(Collectors.toList()));
        }
        
        for (int i = 0; i < posList.size(); i++) {
            posList.get(i).sort((a, b) -> a.getValue() - b.getValue());
            res.add(posList.get(i).stream().map(a -> a.getKey()).collect(Collectors.toList()));
        }
        return res;
    }
}

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/**
 * 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 {
    int min = 0;
    int max = 0;
    public List<List<Integer>> verticalOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        computeRange(root, 0);
        for (int i = min; i <= max; i++) {
            res.add(new ArrayList<>());
        }
        Queue<Integer> qid = new ArrayDeque<>();
        qid.add(- min);
        Queue<TreeNode> q = new ArrayDeque<>();
        q.add(root);
        while (q.size() > 0) {
            TreeNode node = q.poll();
            int id = qid.poll();
            res.get(id).add(node.val);
            
            if (node.left != null) {
                qid.add(id - 1);
                q.add(node.left);
            }
            if (node.right != null) {
                qid.add(id + 1);
                q.add(node.right);
            }
        }
        return res;
    }
    
    
    void computeRange(TreeNode node, int id) {
        if (node == null) return;
        min = Math.min(min, id);
        max = Math.max(max, id);
        computeRange(node.left, id - 1);
        computeRange(node.right, id + 1);
    }
}