board =
[
  ['A','B','C','E'],
  ['S','F','C','S'],
  ['A','D','E','E']
]

给定 word = "ABCCED", 返回 true
给定 word = "SEE", 返回 true
给定 word = "ABCB", 返回 false

board 和 word 中只包含大写和小写英文字母。
1 <= board.length <= 200
1 <= board[i].length <= 200
1 <= word.length <= 10^3

int x[]= {-1,0,1,0};
int y[]= {0,-1,0,1};
boolean isvalue=false;
boolean jud[][];
public boolean exist(char[][] board, String word) {
	jud=new boolean[board.length][board[0].length];
	if(word.length()>board.length*board[0].length)return false;
	char words[]=word.toCharArray();
	for(int i=0;i<board.length;i++)
	{
		for(int j=0;j<board[i].length;j++)
		{
			jud[i][j]=true;
			if(words[0]==board[i][j])
			  dfs(board, 1, words, i, j);
			jud[i][j]=false;
			if(isvalue)return true;
		}
	}	
	return false;
   }
private void dfs(char[][] board,int index, char words[],int x1,int y1) {
	// TODO Auto-generated method stub
	if(isvalue)return;
	if(index==words.length)
	{
		isvalue=true;
	}
	else 
	{
		for(int i=0;i<4;i++)
		{
			int xnew=x1+x[i],ynew=y1+y[i];
			if(xnew>=0&&xnew<board.length&&ynew>=0&&ynew<board[0].length)
			{
				if(!jud[xnew][ynew]&&words[index]==board[xnew][ynew])
				{
					jud[xnew][ynew]=true;
					dfs(board, index+1, words, xnew, ynew);
					jud[xnew][ynew]=false;
				}
			}
		}
	}
}