【Codeforces 375A】Divisible by Seven

【链接】 我是链接,点我呀:)
【题意】

让你把一个包含数字1,6,8,9的数字重新组合,使得组合成的数字能被7整除

【题解】

我们先提取出来1,6,8,9各1个
然后把剩余的len-4个数字除了0之外放在前面
那么这len-4个除了0之外的数字组成的十进制数字对7的取余结果肯定是0~6之间。
然后我们用这个取余结果,和1,6,8,9的所有全排列一个一个去试,尝试把排列出来的结果放在这len-4个数字的后面
最后发现取余结果为0~6都能找到一个1,6,8,9的排列,使得它放在他们后面,然后取余结果为0
最后把剩余的0放在1,6,8,9的后面就好。

【代码】

import java.io.*;
import java.util.*;

public class Main {
    
    
    static InputReader in;
    static PrintWriter out;
        
    public static void main(String[] args) throws IOException{
        //InputStream ins = new FileInputStream("E:\\rush.txt");
        InputStream ins = System.in;
        in = new InputReader(ins);
        out = new PrintWriter(System.out);
        //code start from here
        new Task().solve(in, out);
        out.close();
    }
    
    static int N = 50000;
    static class Task{
        
        int cnt[] = new int[10];
        int a[] = {0,1,6,8,9};
        int b[] = new int[10];
        boolean bo[] = new boolean[5];
        int n;
        String s;
        int cur = 0;
        
        boolean dfs(int dep,int now) {
            if (dep==5) {
                int temp = now;
                for (int i = 1;i <= 4;i++) {
                    temp = temp*10 + b[i];
                    temp = temp%7;
                }
                if (temp==0) {
                    for (int i = 1;i <= 4;i++) {
                        out.print(b[i]);
                    }
                    return true;
                }
                return false;
            }
            for (int i = 1;i <= 4;i++)
                if (!bo[i]) {
                    bo[i] = true;
                    b[dep] = a[i];
                    if (dfs(dep+1,now)) return true;
                    bo[i] = false;
                }
            return false;
        }
        
        public void solve(InputReader in,PrintWriter out) {
            s = in.next();
            for (int i = 0;i < (int)s.length();i++) {
                int key = s.charAt(i)-'0';
                if (key==1 && cnt[1]==0) {
                    cnt[1] = 1;
                }else if (key==6 && cnt[6]==0){
                    cnt[6] = 1;
                }else if (key==8 && cnt[8]==0) {
                    cnt[8] = 1;
                }else if (key==9 && cnt[9]==0) {
                    cnt[9] = 1;
                }else {
                    cnt[key]++;
                    if (key!=0) {
                        out.print(key);
                        cur = cur * 10 + key;
                        cur = cur % 7;
                    }
                }
            }
            dfs(1,cur);
            
            
            for (int i = 1;i <= cnt[0];i++) {
                out.print(0);
            }
        }
    }

    

    static class InputReader{
        public BufferedReader br;
        public StringTokenizer tokenizer;
        
        public InputReader(InputStream ins) {
            br = new BufferedReader(new InputStreamReader(ins));
            tokenizer = null;
        }
        
        public String next(){
            while (tokenizer==null || !tokenizer.hasMoreTokens()) {
                try {
                tokenizer = new StringTokenizer(br.readLine());
                }catch(IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }
        
        public int nextInt() {
            return Integer.parseInt(next());
        }
    }
}