View Compilation Error

0_0_33509838_24839.cpp: In function 'int main()':
0_0_33509838_24839.cpp:17:32: error: 'strlen' was not declared in this scope
             len = strlen(str[i]);
                                ^
0_0_33509838_24839.cpp:24:28: error: 'strlen' was not declared in this scope
         len = strlen(str[f]);
                            ^
0_0_33509838_24839.cpp:40:41: error: 'strstr' was not declared in this scope
                     if(!strstr(str[k],s1) && !strstr(str[k],s2))
                                         ^