答案

public class Solution {
    boolean [][]visit;
    char[][]board;
    int ROW;
    int COL;
    public boolean exist(int row,int col,String word)
    {
        if(board[row][col]!=word.charAt(0))
        {
            return false;
        }
        if(word.length()==1)
        {
            return true;
        }
        boolean result=false;
        visit[row][col]=true;
        String remaining=word.substring(1);
        if(row>0&&!visit[row-1][col])
        {
            result=exist(row-1,col,remaining);
        }
        if(!result&&row<ROW-1&&!visit[row+1][col])
        {
            result=exist(row+1,col,remaining);
        }
        if(!result&&col>0&&!visit[row][col-1])
        {
            result=exist(row,col-1,remaining);
        }
        if(!result&&col<COL-1&&!visit[row][col+1])
        {
            result=exist(row,col+1,remaining);
        }
        visit[row][col]=false;
        return result;
        
    }
    public boolean exist(char[][] board, String word) {
        if(word==null||word.length()==0)
        {
            return true;
        }
        if(board==null)
        {
            return false;
        }
        this.board=board;
        ROW=board.length;
        COL=board[0].length;
        visit=new boolean[ROW][COL];
        for(int i=0;i<ROW;i++)
        {
            for(int j=0;j<COL;j++)
            {
                visit[i][j]=false;
            }
        }
        for(int i=0;i<ROW;i++)
        {
            for(int j=0;j<COL;j++)
            {
                if(exist(i,j,word)){
                        return true;
                }
            }
        }
        return false;
    }
}