第六届蓝桥杯Java A——九数分三组

1~9的数字可以组成3个3位数，设为：A,B,C, 现在要求满足如下关系：
B = 2 * A
C = 3 * A

请你写出A的所有可能答案，数字间用空格分开，数字按升序排列。

import java.util.Set;
import java.util.TreeSet;

public class Main {

    static int[] book = new int[9];
    static int[] res = new int[9];
    static StringBuilder a;
    static StringBuilder b;
    static StringBuilder c;
    static Set<String> set = new TreeSet<String>();

    public static void main(String[] args) {
        dfs(0);
        for (String s : set)
            System.out.print(s + " ");
    }

    static void dfs(int idx) {
        if (idx == 9) {
            a = new StringBuilder("");
            b = new StringBuilder("");
            c = new StringBuilder("");
            if (check())
                set.add(a.toString());
            return;
        }
        for (int i = 1; i <= 9; i++) {
            if (book[i - 1] == 0) {
                book[i - 1] = 1;
                res[idx] = i;
                dfs(idx + 1);
                book[i - 1] = 0;
            }
        }
    }

    private static boolean check() {
        a.append(res[0]).append(res[1]).append(res[2]);
        b.append(res[3]).append(res[4]).append(res[5]);
        c.append(res[6]).append(res[7]).append(res[8]);
        int numa = Integer.parseInt(a.toString());
        int numb = Integer.parseInt(b.toString());
        int numc = Integer.parseInt(c.toString());
        return (numb == numa << 1) && (numc == numa * 3);
    }
}