View Compilation Error

0_0_17772530_28019.cpp: In function 'bool cmp(const node&, const node&)':
0_0_17772530_28019.cpp:17:30: error: 'strcmp' was not declared in this scope
   return strcmp(a.time,b.time)<0;   //题目相等时，按时间从小到达排序
                              ^
0_0_17772530_28019.cpp: In function 'int main()':
0_0_17772530_28019.cpp:34:31: error: 'strcpy' was not declared in this scope
    strcpy(index[i],stu[i].time);
                               ^
0_0_17772530_28019.cpp:37:23: error: 'memset' was not declared in this scope
   memset(x,0,sizeof(x));           //相同题目数的人数
                       ^
0_0_17772530_28019.cpp:53:35: error: 'strcmp' was not declared in this scope
     if(strcmp(index[i],stu[j].time)==0)
                                   ^