【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());
        }
    }
}