Word Search

public class Solution {
    public boolean exist(char[][] board, String word) {
        if(!isValidBoard(board) || word == null || word.length() == 0) {
            return false;
        }
        
        boolean[][] visitedMap = new boolean[board.length][board[0].length];
        for(int i = 0; i < board.length; i++) {
            for(int j = 0; j < board[i].length; j++) {
                if(contains(board, visitedMap, word, i, j, 0)) {
                    return true;
                }
            }
        }
        
        return false;
    }
    
    private boolean isValidBoard(char[][] board) {
        if(board == null || board.length == 0 || board[0] == null || board[0].length == 0) {
            return false;
        } else {
            return true;
        }
    }
    
    private boolean contains(char[][] board, boolean[][] visitedMap, String word, int i, int j, int index) {
        if(index >= word.length() || visitedMap[i][j] == true) {
            return false;
        } else if(index == word.length()-1 && board[i][j] == word.charAt(index)) {
            return true;
        } 
        
        if(board[i][j] == word.charAt(index)) {
            visitedMap[i][j] = true;
            
            // upper
            if(i > 0 && contains(board, visitedMap, word, i-1, j, index+1)) {
                return true;
            }
            
            // down
            if(i < board.length-1 && contains(board, visitedMap, word, i+1, j, index+1)) {
                return true;
            }
            
            // left
            if(j > 0 && contains(board, visitedMap, word, i, j-1, index+1)) {
                return true;
            }
            
            // right
            if(j < board[i].length-1 && contains(board, visitedMap, word, i, j+1, index+1)) {
                return true;
            }
            
            visitedMap[i][j] = false;
        }
        
        return false;
    }
    
}