Lintcode 123. Word Search

public class Solution {

    private static final int[][] DELTA_DIMS = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};

    /**
     * @param board: A list of lists of character
     * @param word: A string
     * @return: A boolean
     */
    public boolean exist(char[][] board, String word) {
        if(word == null) {
            return false;
        }

        int m = (board == null) ? 0 : board.length;
        int n = (m == 0) ? 0 : board[0].length;

        for(int i = 0; i < m; i++) {
            for(int j = 0; j < n; j++) {
                if(exist(board, i, j, word, 0)) {
                    return true;
                }
            }
        }
        return false;
    }

    private boolean exist(char[][] board, int x, int y, String word, int wordI) {
        if(wordI == word.length()) {
            return true;
        }

        if(wordI > word.length()) {
            return false;
        }

        if(!inBound(board, x, y)) {
            return false;
        }

        if(board[x][y] != word.charAt(wordI)) {
            return false;
        }

        char origChar = board[x][y];
        board[x][y] = 0;

        for(int[] deltaDim : DELTA_DIMS) {
            int nx = x + deltaDim[0];
            int ny = y + deltaDim[1];

            if(exist(board, nx, ny, word, wordI + 1)) {
                board[x][y] = origChar;
                return true;
            }
        }

        board[x][y] = origChar;
        return false;
    }

    private boolean inBound(char[][] board, int x, int y) {
        int m = (board == null) ? 0 : board.length;
        int n = (m == 0) ? 0 : board[0].length;

        return x >= 0 && x < m &&
            y >= 0 && y < n;
    }
}