C++ :

#include<iostream>
#include<string>
using namespace std;
int mod7(string s){
    int sum=0;
    for(int i=0;i<s.size();i++){
        sum=(sum*10+(s[i]-'0'))%7;
    }
    if(sum%7==0) return 1;
    else return 0;
}

int main()
{
    int t;
    string s;
    while(cin>>s){
        if(mod7(s)) cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
    return 0;
}

Pascal :

const
 b:array[1..6]of longint=(1,3,2,-1,-3,-2);
var
 st:string;
 a:array[0..110]of longint;
 i,j,n,s:longint;
begin
 while not eof do
  begin
  readln(st); s:=0;
 if st<>'' then
 begin
 for i:= 1 to length(st) do
  val(st[i],a[i]);
 j:=1;
 for i:= length(st) downto 1 do
  begin
   s:=s+b[j]*a[i];
   if j=6 then j:=1 else j:=j+1;
  end;
 if s mod 7=0 then writeln('YES') else writeln('NO');
 end;
 end;
end.