不需要暴力求解，其实你可以把它想象成一个0 1字符串，然后就行求解即可。
public static void main(String args[]) {
Scanner reader = new Scanner(System.in);
while (reader.hasNextLine()) {
int num = Integer.parseInt(reader.nextLine());
for (int i = 0; i < num; i++) {
int count = Integer.parseInt(reader.nextLine());
if (count == 1) {
System.out.println(4);
continue;
}
if (count == 2) {
System.out.println(7);
continue;
}
int a = 1;
while (Math.pow(2, a) - 2 < count) {
a++;
}
count = (int) (count - Math.pow(2, a - 1) + 2);
count--;
StringBuilder sb = new StringBuilder();
for (int j = 0; j < a - 1; j++) {
int index = 1 << j;
if ((count & index) == 0) {
sb.append("4");
} else {
sb.append("7");
}
}
System.out.println(sb.reverse().toString());
}
}
}
