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
提示：
1 <= board.length <= 200
1 <= board[i].length <= 200
代码：
class Solution {
public boolean exist(char[][] board, String word) {
if (board == null || board[0] == null || board.length == 0 || board[0].length == 0 || word == null || word.equals("")) {
return false;
}
boolean visited[][] = new boolean[board.length][board[0].length];
for(int i=0;i<board.length;i++)
{
for(int j=0;j<board[0].length;j++)
{
if(board[i][j]==word.charAt(0))
{
if(find(board,word,0,i,j,visited)==true)
{
return true;
}
}
}
}
return false;
}
public boolean find(char[][] board,String word,int index,int i,int j, boolean visited[][])
{
if(index==word.length())
{
return true;
}
if (i < 0 || j < 0 || i == board.length || j == board[0].length || visited[i][j] || board[i][j] != word.charAt(index)) {
return false;
}
visited[i][j] = true;
boolean x = find(board,word,index+1,i-1,j,visited)||find(board,word,index+1,i,j-1,visited)||find(board,word,index+1,i+1,j,visited)||find(board,word,index+1,i,j+1,visited);
visited[i][j] = false;
return x;
}
}
