Solution.java

class Solution {
    public List<String> printKMoves(int K) {
        int x1 = 0, y1 = 0, x2 = 0, y2 = 0;
        int[] dirs = {0, 1, 0, -1, 0};
        String d = "RDLU";
        int x = 0, y = 0, p = 0;
        Set<List<Integer>> black = new HashSet<>();
        while (K-- > 0) {
            List<Integer> t = List.of(x, y);
            if (black.add(t)) {
                p = (p + 1) % 4;
            } else {
                black.remove(t);
                p = (p + 3) % 4;
            }
            x += dirs[p];
            y += dirs[p + 1];
            x1 = Math.min(x1, x);
            y1 = Math.min(y1, y);
            x2 = Math.max(x2, x);
            y2 = Math.max(y2, y);
        }
        int m = x2 - x1 + 1;
        int n = y2 - y1 + 1;
        char[][] g = new char[m][n];
        for (char[] row : g) {
            Arrays.fill(row, '_');
        }
        for (List<Integer> t : black) {
            int i = t.get(0) - x1;
            int j = t.get(1) - y1;
            g[i][j] = 'X';
        }
        g[x - x1][y - y1] = d.charAt(p);
        List<String> ans = new ArrayList<>();
        for (char[] row : g) {
            ans.add(String.valueOf(row));
        }
        return ans;
    }
}