987. Vertical Order Traversal of a Binary Tree

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Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[9],[3,15],[20],[7]]
Explanation:
Column -1: Only node 9 is in this column.
Column 0: Nodes 3 and 15 are in this column in that order from top to bottom.
Column 1: Only node 20 is in this column.
Column 2: Only node 7 is in this column.

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Example 2:

Input: root = [1,2,3,4,5,6,7]
Output: [[4],[2],[1,5,6],[3],[7]]
Explanation:
Column -2: Only node 4 is in this column.
Column -1: Only node 2 is in this column.
Column 0: Nodes 1, 5, and 6 are in this column.
          1 is at the top, so it comes first.
          5 and 6 are at the same position (2, 0), so we order them by their value, 5 before 6.
Column 1: Only node 3 is in this column.
Column 2: Only node 7 is in this column.

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Example 3:

Input: root = [1,2,3,4,6,5,7]
Output: [[4],[2],[1,5,6],[3],[7]]
Explanation:
This case is the exact same as example 2, but with nodes 5 and 6 swapped.
Note that the solution remains the same since 5 and 6 are in the same location and should be ordered by their values.

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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 {
  List < List < Integer >> verticalTraversal(TreeNode root) {

    if (root == null) return Collections.emptyList();

    List<List<Node>> posList = new ArrayList<>();
    List<List<Node>> negList = new ArrayList<>();
    dfs(root, 0, 0, negList, posList);
    return mergeAndSort(negList, posList);
  }

  class Node {
    int val;
    int row;
    
    public Node(int val, int row) {
        this.val = val;
        this.row = row;
    }  
  }

  void dfs(TreeNode node, int row, int col, List<List<Node>> negList, List<List<Node>> posList) {

    if (node == null) return;

    if (col >= 0) {
      if (posList.size() == col) {
        posList.add(new ArrayList < >());
      }
      posList.get(col).add(new Node(node.val, row));

    } else {
      int posCol = -col - 1;
      if (negList.size() == posCol) {
        negList.add(new ArrayList<>());
      }
      negList.get(posCol).add(new Node(node.val, row));
    }

    // (row + 1, col - 1) left
    dfs(node.left, row + 1, col - 1, negList, posList);

    // and (row + 1, col + 1) right
    dfs(node.right, row + 1, col + 1, negList, posList);

  }

  List<List<Integer>> mergeAndSort(List<List<Node>> negList, List<List<Node>> posList) {
    List<List<Integer>> res = new ArrayList<>();

    for (int i = negList.size() - 1; i >= 0; i--) {
      List<Node> colList = negList.get(i);
      colList.sort((a, b) -> a.row == b.row ? a.val - b.val : a.row - b.row);
      res.add(buildList(colList)); 
    }

    for (int i = 0; i < posList.size(); i++) {
      List<Node> colList = posList.get(i);
      colList.sort((a, b) -> a.row == b.row ? a.val - b.val : a.row - b.row);
      res.add(buildList(colList)); 
    }

    return res;
  }
    
  List<Integer> buildList(List<Node> list) {
      return list.stream().map(a -> a.val).collect(Collectors.toList());
  }  
}

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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>> verticalTraversal(TreeNode root) {
        List<List<NodeValue>> res = new ArrayList<>();
        computeRange(root, 0);
        for (int i = min; i <= max; i++) {
            res.add(new ArrayList<>());
        }
        Queue<NodeValue> q = new ArrayDeque<>();
        q.add(new NodeValue(root, 0, -min));
        while (q.size() > 0) {
            NodeValue nodeVal = q.poll();
            res.get(nodeVal.col).add(nodeVal);
            
            if (nodeVal.node.left != null) {
                q.add(new NodeValue(nodeVal.node.left, nodeVal.row + 1, nodeVal.col - 1));
            }
            if (nodeVal.node.right != null) {
                q.add(new NodeValue(nodeVal.node.right, nodeVal.row + 1, nodeVal.col + 1));
            }
        }
        for (int i = 0; i < res.size(); i++) {
            Collections.sort(res.get(i), (a,b) -> {
                int c = Integer.compare(a.row, b.row);
                if (c != 0) return c;
                return Integer.compare(a.node.val, b.node.val);
            });
        }
        List<List<Integer>> resInt = new ArrayList();
        for (List<NodeValue> nodes : res) {
            List<Integer> rows = new ArrayList<>();
            for (NodeValue n: nodes) {
                rows.add(n.node.val);
            }
            resInt.add(rows);
        }
        return resInt;
    }
    
    class NodeValue{
        TreeNode node;
        int row;
        int col;
        
        NodeValue(TreeNode node, int row, int col) {
            this.node = node;
            this.row = row;
            this.col = col;
        }
    }
    
    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);
    }
}