class Solution {    boolean[][] used;    public boolean exist(char[][] board, String word) {        used = 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 (board[i][j] == word.charAt(0))                    if(DFS(board, word, i, j, 0))                        return true;                else                    continue;            }        }        return false;    }        public boolean DFS(char[][] board, String word, int i, int j, int index) {        if (i < 0 || j < 0 || i >= board.length || j >= board[i].length || word.charAt(index) != board[i][j] || used[i][j]) {            return false;        }        else if (index == word.length() - 1) {            return true;        } else {            used[i][j] = true;            if(DFS(board, word, i, j+1, index+1) || DFS(board, word, i+1, j, index+1) || DFS(board, word, i, j-1, index+1) || DFS(board, word, i-1, j, index+1)) {                return true;            } else {                used[i][j] = false;                return false;            }        }    }    }