My 19ms accepted C++ code
   class Solution {\n        public:\n        \t bool exist(vector<vector<char> > &board, string word) {\n        \t\t m=board.size();\n        \t\t n=board[0].
       class Solution {\n        public:\n        \t bool exist(vector<vector<char> > &board, string word) {\n        \t\t m=board.size();\n        \t\t n=board[0].size();\n                for(int x=0;x<m;x++)\n                    for(int y=0;y<n;y++)\n                    {\n        \t\t\t\tif(isFound(board,word.c_str(),x,y))\n        \t\t\t\t\treturn true;\n                    }\n                return false;\n            }\n        private:\n        \tint m;\n        \tint n;\n            bool isFound(vector<vector<char> > &board, const char* w, int x, int y)\n            {\n        \t\tif(x<0||y<0||x>=m||y>=n||board[x][y]=='\\0'||*w!=board[x][y])\n        \t\t\treturn false;\n                if(*(w+1)=='\\0')\n                    return true;\n        \t\tchar t=board[x][y];\n        \t\tboard[x][y]='\\0';\n        \t\tif(isFound(board,w+1,x-1,y)||isFound(board,w+1,x+1,y)||isFound(board,w+1,x,y-1)||isFound(board,w+1,x,y+1))\n        \t\t\treturn true; \n        \t\tboard[x][y]=t;\n                return false;\n            }\n        };
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