矩阵中的路径（单词搜索）

请设计一个函数，用来判断在一个矩阵中是否存在一条包含某字符串所有字符的路径。路径可以从矩阵中的任意一格开始，每一步可以在矩阵中向左、右、上、下移动一格。如果一条路径经过了矩阵的某一格，那么该路径不能再次进入该格子。例如，在下面的3×4的矩阵中包含一条字符串“bfce”的路径（路径中的字母用加粗标出）。

[["a","b","c","e"],
["s","f","c","s"],
["a","d","e","e"]]

但矩阵中不包含字符串“abfb”的路径，因为字符串的第一个字符b占据了矩阵中的第一行第二个格子之后，路径不能再次进入这个格子 

示例 1：

输入：board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "ABCCED"
输出：true

示例 2：

输入：board = [["a","b"],["c","d"]], word = "abcd"
输出：false

public boolean exist(char[][] board, String word) {
        char[] words = word.toCharArray();
        int row = board.length;  //行数
        int col = board[0].length;//列数
        for(int i=0;i<row;i++){
            for(int j = 0;j<col;j++){
                if(dfs(board,words,i,j,0)){
                    return true;
                }
            }
        }       
        return false;
    }

   boolean dfs(char[][] board,char[] words,int i,int j,int k){
        if(i<0 || i>=board.length || j<0 || j>=board[0].length || board[i][j]!=words[k]){
            return false;
        }
        if(k==words.length-1){
            return true;
        }

        char tmp = board[i][j];
        board[i][j]='/';

        boolean flag = dfs(board, words, i + 1, j, k + 1) || dfs(board, words, i - 1, j, k + 1) ||
                      dfs(board, words, i, j + 1, k + 1) || dfs(board, words, i , j - 1, k + 1);

        board[i][j]=tmp;
        return flag;
    }