给定一个 m x n 二维字符网格 board 和一个字符串单词 word 。如果 word 存在于网格中，返回 true ；否则，返回 false 。
单词必须按照字母顺序，通过相邻的单元格内的字母构成，其中“相邻”单元格是那些水平相邻或垂直相邻的单元格。同一个单元格内的字母不允许被重复使用。
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","E"],["S","F","C","S"],["A","D","E","E"]], word = "SEE"
输出：true
示例 3：
输入：board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "ABCB"
输出：false
class Solution {
public:bool exist(vector<vector<char>>& board, string word) {int x=board.size();int y=board[0].size();for(int i=0;i<x;i++){for(int j=0;j<y;j++){if(backtrack(board,word,i,j,0)){return true;}}}return false;}bool backtrack(vector<vector<char>>& board, string word,int i,int j,int k){if(k==word.size()){return true;}if(i<0||j<0||i>=board.size()||j>=board[0].size()||board[i][j]!=word[k]){return false;}char a=board[i][j];board[i][j]='\0';bool re=(backtrack(board,word,i-1,j,k+1)||backtrack(board,word,i+1,j,k+1)||backtrack(board,word,i,j+1,k+1)||backtrack(board,word,i,j-1,k+1));board[i][j]=a;return re;}
};
