View Compilation Error

0_0_20611377_3337.cpp: In function 'int main()':
0_0_20611377_3337.cpp:3:247: error: 'strlen' was not declared in this scope
     using namespace std; char date[1000][1000];char strings[2000]; int main (){     int num;    int i,j;    int len;    int pos=0;    while(cin>>num)    {         pos=0;        for(i=0;i<num;i++)            cin>>date[i];        len=strlen(date[0]);        int flag=0;        for(j=0;j<len;j++)        {             for(i=0;i<num;i++)            {                if(date[i][j]!='_')                    flag=1;                            if(flag==1)                                  {                    strings[pos]=date[i][j];                    pos++;                }            }         }        strings[pos]='\0';        reverse(strings,strings+pos);        for(i=0;i<pos;i++)        {            if(strings[i]=='_')            {                printf(" ");                strings[i]=' ';                continue;            }            else if(strings[i]=='\\')            {                cout<<'\n';                continue;            }            else                 cout<<strings[i];        }        cout<<endl<<endl;    }    return 0;}
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