View Compilation Error

0_0_13760624_7007.cpp: In function 'int main()':
0_0_13760624_7007.cpp:26:35: error: 'memset' was not declared in this scope
         memset(used,0,sizeof(used));  
                                   ^
0_0_13760624_7007.cpp:32:26: error: 'strlen' was not declared in this scope
             len=strlen(ch);  
                          ^
0_0_13760624_7007.cpp:42:30: error: 'strlen' was not declared in this scope
                 len=strlen(ch);  
                              ^