public boolean exist(char[][] board, String word) {
        int m = board.length;
        int n = board[0].length;
        boolean[][] visited = new boolean[m][n];
        int len = word.length();
        int k = 0;
        for(int i = 0;i<m;i++){
            for(int j = 0;j<n;j++){
                
 
if(board[i][j] == word.charAt(0)){
                    if(find(i,j,m,n,len,k,board,word,visited)){
                        return true;
                    }
                }
            }
        }
        return false;
    }

    private boolean find(int i, int j, int m, int n,int len, int k, char
 
[][] board, String word, boolean[][] visited) {
        //搜尋該位置的上(i-1,j)下(i+1,j)左(i,j-1)右(i,j+1)，看是否能找到下一個字元
        if(i<0 || i>=m || j<0 || j>=n) return false;
        if(!visited[i][j]){
            if(board[i][j] == word.charAt(k)) {
                if(k == len-1) return true;
                
 
if(i>0){
                    visited[i][j] = true;
                    if(find(i - 1, j, m, n, len, k + 1,board,word,visited)){
                        return true;
                    }
                    visited[i][j] = false;
                }
                if(i<m-1){
                    visited[i][j] = true;
                    if(find(i + 1, j, m, n, len, k + 1,board,word,visited)){
                        return true;
                    }
                    visited[i][j] = false;
                }
                if(j>0){
                    visited[i][j] = true;
                    if(find(i, j - 1, m, n, len, k + 1,board,word,visited)){
                        return true;
                    }
                    visited[i][j] = false;
                }
                if(j<n-1){
                    visited[i][j] = true;
                    if(find(i, j + 1, m, n, len, k + 1,board,word,visited)){
                        return true;
                    }
                    visited[i][j] = false;
                }
            }
        }
        return false;
    }