Run time error on empty input?

Question

Hi, I got a run time error on a empty input[ ]. my code works fine with my own test codes. Any idea of what is happening?

And I use empty char[ ][ ] to test on local machine, it works fine.

My algorithm is to start from 'o's on the edges of the board, then use a iterative DFS to find 'o' clique on the board. Then mark 'x' to other 'o's that are not connected to the edge 'o's.

public static void solve(char[][] board) {
    if(board==null){
        return;
    }

    if(board[0]==null){
        return;
    }

    boolean[][] visited = new boolean[board.length][board.length];

    for (int i = 0; i < board.length; i++) {
        if (board[i][0] == 'o' && visited[i][0] == false) {
            visited[i][0] = true;
            findClique(i, 0, board, visited);
        }
        if (board[i][board[0].length - 1] == 'o'
                && visited[i][board[0].length - 1] == false) {
            visited[i][board[0].length - 1] = true;
            findClique(i, board[0].length - 1, board, visited);
        }

    }

    for (int i = 0; i < board[0].length; i++) {
        if (board[0][i] == 'o'&&visited[0][i] ==false) {
            visited[0][i] = true;
            findClique(0, i, board, visited);
        }
        if (board[board.length - 1][i] == 'o'&&visited[board.length - 1][i] == false) {
            visited[board.length - 1][i] = true;
            findClique(board.length - 1, i, board, visited);
        }

    }


    for (int i = 0; i < board.length; i++) {
        for (int m = 0; m < board[0].length; m++) {
            if(board[i][m]=='o'&& visited[i][m]==false){
                board[i][m] = 'x'; 
            }
        }
    }

    return;
}//

private static void findClique(int x, int y, char[][] board,
        boolean[][] visited) {

    visited[x][y] = false;

    Stack<Integer> stx = new Stack<Integer>();
    Stack<Integer> sty = new Stack<Integer>();
    stx.push(x);
    sty.push(y);

    int xtmp = 0;
    int ytmp = 0;
    while (!stx.isEmpty()) {
        xtmp = stx.pop();
        ytmp = sty.pop();
        visited[xtmp][ytmp] = true;

        if (xtmp > 0) {
            if (board[xtmp - 1][ytmp] == 'o'
                    && visited[xtmp - 1][ytmp] == false) {
                stx.push(xtmp - 1);
                sty.push(ytmp);
            }
        }

        if (xtmp < board.length - 1) {
            if (board[xtmp + 1][ytmp] == 'o'
                    && visited[xtmp + 1][ytmp] == false) {
                stx.push(xtmp + 1);
                sty.push(ytmp);
            }
        }

        if (ytmp > 0) {
            if (board[xtmp][ytmp - 1] == 'o'
                    && visited[xtmp][ytmp - 1] == false) {
                stx.push(xtmp);
                sty.push(ytmp - 1);
            }
        }

        if (ytmp < board[0].length - 1) {
            if (board[xtmp][ytmp + 1] == 'o'
                    && visited[xtmp][ytmp + 1] == false) {
                stx.push(xtmp);
                sty.push(ytmp + 1);
            }
        }

    }

}