View Compilation Error

0_0_30226255_1468.cpp: In function 'int main()':
0_0_30226255_1468.cpp:14:24: error: 'strlen' was not declared in this scope
             if(strlen(a)==1)
                        ^
0_0_30226255_1468.cpp:19:31: error: 'strlen' was not declared in this scope
             for(i=0;i<strlen(a)&&t!=1;i++)
                               ^
0_0_30226255_1468.cpp:22:36: error: 'strcmp' was not declared in this scope
                 if(strcmp(ch,a[i+1])==0&&strlen(a)>1)
                                    ^