For example,
Given board =
[
  ['A','B','C','E'],
  ['S','F','C','S'],
  ['A','D','E','E']
]
word = "ABCCED", -> returns true,
word = "SEE", -> returns true,
word = "ABCB", -> returns false.

private static final int[][] direction = {{1,0},{-1,0},{0,1},{0,-1}};
    
    public boolean exist(char
 
[][] board,String word){
        if(word == null || word.length() == 0){
            return true;
        }
        if(board == null || board.length == 0 || board[0].length==0){
            return false;
        }
        boolean[][] hasVisited = new boolean[board.length][board[0].length];
        for(int
 
 r=0;r<board.length;r++){
            for(int c=0;c<board[0].length;c++){
                if(backtracking(0,r,c,hasVisited,board,word)){
                    return true;
                }
            }
        }
        return false;
    }
    
    private boolean backtracking(int curLen,int r,int
 
 c,boolean[][] visited,final char[][] board,final String word){
        if(curLen == word.length()){
            return true;
        }
        if(r<0||r>=board.length||c<0||c>=board[0].length || board[r][c]!=word.charAt(curLen)||visited[r][c]){
            return false;
        }
        visited[r][c] = true;
        
        for(int[] d :direction){
            if(backtracking(curLen+1, r, c, visited, board, word)){
                return true;
            }
        }
        
        visited[r][c] = false;
        
        return false;
    }