import java.util.regex.Pattern;
public class BinaryDivisibleBy7 {
// Check if a string contains only '0' and '1'
static boolean isBinary(String s) {
// Java compiles regex patterns once when declared static final
final Pattern binary = Pattern.compile("^[01]+$");
return binary.matcher(s).matches();
}
// In decimal: reading "123" means
// remainder = remainder * 10 + digit
// In binary: reading "1011" means
// remainder = remainder * 2 + bit
// 0 (0*2 + 0) % 7 0
// 0 (0*2 + 0) % 7 0
// 0 (0*2 + 0) % 7 0
// 1 (0*2 + 1) % 7 1
// 1 (1*2 + 1) % 7 3
// 1 (3*2 + 1) % 7 0
// 0 (0*2 + 0) % 7 0
// 0 (0*2 + 0) % 7 0
// Compute binary string modulo 7 without overflow
static boolean divisibleBy7(String s) {
int remainder = 0;
for (int i = 0; i < s.length(); i++) {
char c = s.charAt(i);
remainder = (remainder * 2 + (c - '0')) % 7;
}
return remainder == 0;
}
public static void main(String[] args) {
String s = "00011100"; // 28
if (!isBinary(s)) {
System.out.println("Not a binary string");
return;
}
if (divisibleBy7(s)) {
System.out.println("Divisible by 7");
} else {
System.out.println("Not divisible by 7");
}
}
}
/*
run:
Divisible by 7
*/
