https://leetcode.com/problems/word-search

class Solution {
    public boolean exist(char[][] board, String word) {
        if(board == null || board.length == 0 || board[0].length == 0 || word == null || word.length() == 0) return false;
        int rows = board.length;
        int cols = board[0].length;
        for(int i=0; i < rows; i++) {
            for(int j = 0; j < cols; j++){
                if(search(board, word, i, j, 0))
                    return true;
            }
        } 
        return false;     
    }
    
    private boolean search(char[][] board, String word, int i, int j, int pos) {
        if(pos == word.length()) return true;
        
        if(board[i][j] != word.charAt(pos))
            return false;
        int rows = board.length;
        int cols = board[0].length;
        board[i][j] = '.';
        if(pos == word.length()-1) return true;
        if (( i+1 < rows && board[i+1][j] != '.' && search(board, word,  i + 1, j, pos+1) ) || 
            ( i-1 >= 0 && board[i-1][j] != '.' && search(board, word,  i - 1, j, pos+1) ) ||
            ( j+1 < cols && board[i][j+1] != '.' && search(board, word,  i, j + 1, pos+1) ) ||
            ( j-1 >=0 && board[i][j-1] != '.' && search(board, word, i, j - 1, pos+1) ) ) {
                return true;
        }
        board[i][j] = word.charAt(pos);
        return false;
    }
}