863. all nodes distance k in binary tree

 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class  {
    Map<TreeNode, HashSet<TreeNode>> map = new HashMap<>();
    public List<Integer> distanceK(TreeNode root, TreeNode target, int K) {
        List<Integer> list = new ArrayList<>();
        if (root == null) return list;
        buildMap(root);
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(target);
        Set<TreeNode> visited = new HashSet<>();
        visited.add(target);
        boolean found = false;
        int dis = 0;
        while (!q.isEmpty()) {
            int len = q.size();
            for (int size = 0; size < len; size++) {
                TreeNode cur = q.poll();
                if (dis == K) {
                    list.add(cur.val);
                    found = true;
                }else {
                    if (!map.get(cur).isEmpty()) {
                        for (TreeNode t : map.get(cur)) {
                            if (!visited.contains(t)) {
                                q.offer(t);
                                visited.add(t);
                            }
                        }
                    }
                }
            }
            if (found) return list;
            dis++;
        }
        return list;
    }
    public void buildMap(TreeNode node) {
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(node);
        map.put(node, new HashSet<TreeNode>());
        while (!q.isEmpty()) {
            TreeNode cur = q.poll();
            if (cur.left != null) {
                map.put(cur.left, new HashSet<TreeNode>());
                map.get(cur).add(cur.left);
                map.get(cur.left).add(cur);
                q.offer(cur.left);
            }
            if (cur.right != null) {
                map.put(cur.right, new HashSet<TreeNode>());
                map.get(cur).add(cur.right);
                map.get(cur.right).add(cur);
                q.offer(cur.right);
            }
        }
    }
}