View Compilation Error

0_0_18238110_15610.cpp: In function 'bool com(Node&, Node&)':
0_0_18238110_15610.cpp:11:28: error: 'strcmp' was not declared in this scope
  return (strcmp(a.str,b.str)<0);
                            ^
0_0_18238110_15610.cpp: In function 'int minexp(char*)':
0_0_18238110_15610.cpp:15:20: error: 'strlen' was not declared in this scope
  int len=strlen(str);
                    ^
0_0_18238110_15610.cpp: In function 'int main()':
0_0_18238110_15610.cpp:42:22: error: 'strlen' was not declared in this scope
    int len=strlen(now);
                      ^
0_0_18238110_15610.cpp:56:41: error: 'strcmp' was not declared in this scope
    if(strcmp(nodes[i-1].str,nodes[i].str)!=0){
                                         ^