View Compilation Error

0_0_29279438_1504.cpp: In function 'int main()':
0_0_29279438_1504.cpp:39:29: error: 'memset' was not declared in this scope
   memset(res, 0, sizeof(res));
                             ^
0_0_29279438_1504.cpp:84:30: error: 'reverse' was not declared in this scope
    reverse(res, res + len + 1);
                              ^
0_0_29279438_1504.cpp:94:26: error: 'reverse' was not declared in this scope
    reverse(res, res + len);
                          ^