View Compilation Error

0_0_18104264_11221.cpp: In function 'int main()':
0_0_18104264_11221.cpp:33:37: error: 'memset' was not declared in this scope
         memset(used, 0, sizeof(used));  
                                     ^
0_0_18104264_11221.cpp:37:32: error: 'strlen' was not declared in this scope
             int len = strlen(ch);  
                                ^
0_0_18104264_11221.cpp:46:36: error: 'strlen' was not declared in this scope
                 int len = strlen(ch);  
                                    ^