View Compilation Error

0_0_19588255_13934.cpp: In function 'int huffman()':
0_0_19588255_13934.cpp:17:25: error: 'memset' was not declared in this scope
     memset(a,0,sizeof(a));  
                         ^
0_0_19588255_13934.cpp: In function 'int main()':
0_0_19588255_13934.cpp:44:35: error: 'strcmp' was not declared in this scope
     while(gets(s)&& strcmp("END",s))  
                                   ^
0_0_19588255_13934.cpp:46:19: error: 'strlen' was not declared in this scope
         m=strlen(s);  
                   ^