Leetcode #314. Binary Tree Vertical Order Traversal

[yokostan]

HashMap solution:

/**
 * 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<>();
        if (root == null) return res;

        HashMap<Integer, ArrayList<Integer>> map = new HashMap<>();
        Queue<TreeNode> queue = new LinkedList<>();
        HashMap<TreeNode, Integer> weight = new HashMap<>();
        queue.offer(root);
        weight.put(root, 0);

        int min = 0;
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            int w = weight.get(node);
            if (!map.containsKey(w)) {
                map.put(w, new ArrayList<Integer>());
            }
            map.get(w).add(node.val);

            if (node.left != null) {
                queue.add(node.left);
                weight.put(node.left, w - 1);
            }

            if (node.right != null) {
                queue.add(node.right);
                weight.put(node.right, w + 1);
            }

            min = Math.min(min, w);
        }

        while (map.containsKey(min)) {
            res.add(map.get(min++));
        }

        return res;
    }
}

Here we use HashMap to store <absolute col number, corresponding elements in that level>, weight to store relative col for elements.

The following solution is basically the same, but it used a Queue to store the column values:

/**
 * 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<>();
        if (root == null) return res;

        HashMap<Integer, ArrayList<Integer>> map = new HashMap<>();
        Queue<TreeNode> queue = new LinkedList<>();
        Queue<Integer> cols = new LinkedList<>();

        queue.offer(root);
        cols.add(0);

        int min = 0;
        int max = 0;

        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            int col = cols.poll();
            if (!map.containsKey(col)) {
                map.put(col, new ArrayList<Integer>());
            }
            map.get(col).add(node.val);

            if (node.left != null) {
                queue.add(node.left);
                cols.add(col - 1);
                min = Math.min(min, col - 1);
            }

            if (node.right != null) {
                queue.add(node.right);
                cols.add(col + 1);
                max = Math.max(max, col + 1);
            }
        }

        for (int i = min; i <= max; i++) {
            res.add(map.get(i));
        }

        return res;
    }
}