Solution.java

class Solution {
    private String[] t = new String[6];
    private int[][] board;

    public int slidingPuzzle(int[][] board) {
        this.board = board;
        String start = gets();
        String end = "123450";
        if (end.equals(start)) {
            return 0;
        }
        Set<String> vis = new HashSet<>();
        Deque<String> q = new ArrayDeque<>();
        q.offer(start);
        vis.add(start);
        int ans = 0;
        while (!q.isEmpty()) {
            ++ans;
            for (int n = q.size(); n > 0; --n) {
                String x = q.poll();
                setb(x);
                for (String y : next()) {
                    if (y.equals(end)) {
                        return ans;
                    }
                    if (!vis.contains(y)) {
                        vis.add(y);
                        q.offer(y);
                    }
                }
            }
        }
        return -1;
    }

    private String gets() {
        for (int i = 0; i < 2; ++i) {
            for (int j = 0; j < 3; ++j) {
                t[i * 3 + j] = String.valueOf(board[i][j]);
            }
        }
        return String.join("", t);
    }

    private void setb(String s) {
        for (int i = 0; i < 2; ++i) {
            for (int j = 0; j < 3; ++j) {
                board[i][j] = s.charAt(i * 3 + j) - '0';
            }
        }
    }

    private int[] find0() {
        for (int i = 0; i < 2; ++i) {
            for (int j = 0; j < 3; ++j) {
                if (board[i][j] == 0) {
                    return new int[] {i, j};
                }
            }
        }
        return new int[] {0, 0};
    }

    private List<String> next() {
        int[] p = find0();
        int i = p[0], j = p[1];
        int[] dirs = {-1, 0, 1, 0, -1};
        List<String> res = new ArrayList<>();
        for (int k = 0; k < 4; ++k) {
            int x = i + dirs[k];
            int y = j + dirs[k + 1];
            if (x >= 0 && x < 2 && y >= 0 && y < 3) {
                swap(i, j, x, y);
                res.add(gets());
                swap(i, j, x, y);
            }
        }
        return res;
    }

    private void swap(int i, int j, int x, int y) {
        int t = board[i][j];
        board[i][j] = board[x][y];
        board[x][y] = t;
    }
}