专栏首页机器学习入门POJ 刷题系列:1094. Sorting It All Out

POJ 刷题系列:1094. Sorting It All Out

版权声明:本文为博主原创文章,未经博主允许不得转载。 https://blog.csdn.net/u014688145/article/details/78976235

POJ 刷题系列:1094. Sorting It All Out

传送门:1094. Sorting It All Out

题意:

给出n个字母,m组大小关系,判断是否能够确定n个字母的唯一顺序,其中不能存在环。

思路: 一开始使用了DFS,每当加入一条关系时,更新邻接表,并判断是否存在环以及能否得到唯一的拓扑序,结果一直超时,不知道歪。实际上如果存在唯一的顺序,那么当且仅当只有一个点入度为0,所以当queue同一时刻存在多个入度为0的点时,说明并非唯一。其次,如果最终输出的拓扑序小于n说明存在环。

代码如下:

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Queue;
import java.util.Set;
import java.util.StringTokenizer;

public class Main{

    String INPUT = "./data/judge/201801/P1094.txt";

    public static void main(String[] args) throws IOException {
        new Main().run();
    }

    int n;

    void read() {

        while (true) {
            n = ni();
            int m = ni();
            if (n + m == 0) break;
            List<Integer>[] graph = new ArrayList[n];
            for (int i = 0; i < n; ++i) graph[i] = new ArrayList<Integer>();

            int[] indegree = new int[n];
            boolean hasAns = false;
            hasCircle = false;
            for (int i = 0; i < m; ++i) {
                String ns = ns();
                int u = ns.charAt(0) - 'A';
                int v = ns.charAt(2) - 'A';
                indegree[v] ++;
                graph[u].add(v);
                if (!hasAns) {
                    if (top(graph, indegree)) {
                        out.println("Sorted sequence determined after " + (i + 1) + " relations: " + ans + ".");
                        hasAns = true;
                    }
                    else {
                        if(hasCircle) {
                            out.println("Inconsistency found after "+ (i + 1) +" relations.");
                            hasAns = true;
                        }
                    }
                }
            }
            if (!hasAns)
                out.println("Sorted sequence cannot be determined.");
        }
    }

    String ans = "";
    boolean hasCircle = false;
    boolean top(List<Integer>[] graph, int[] indegree) {
        int[] copy = new int[n];
        System.arraycopy(indegree, 0, copy, 0, n);

        StringBuilder sb = new StringBuilder();
        Queue<Integer> q = new ArrayDeque<Integer>();
        boolean[] vis = new boolean[n];

        for (int i = 0; i < n; ++i) {
            if (copy[i] == 0) {
                q.offer(i);
                vis[i] = true;
            }
        }

        boolean once = true;
        while (!q.isEmpty()) {
            if (q.size() >= 2) once = false;
            int now = q.poll();
            char c = (char) (now + 'A');
            sb.append(c);
            for (int to : graph[now]) {
                copy[to] --;
            }
            for (int i = 0; i < n; ++i) {
                if (!vis[i] && copy[i] == 0) {
                    q.offer(i);
                    vis[i] = true;
                }
            }
        }
        if (sb.length() == n && once) {
            ans = sb.toString();
            return true;
        }
        else if (sb.length() == n && !once) return false;
        hasCircle = true;
        return false;
    }


    FastScanner in;
    PrintWriter out;

    void run() throws IOException {
        boolean oj;
        try {
            oj = ! System.getProperty("user.dir").equals("F:\\oxygen_workspace\\Algorithm");
        } catch (Exception e) {
            oj = System.getProperty("ONLINE_JUDGE") != null;
        }

        InputStream is = oj ? System.in : new FileInputStream(new File(INPUT));
        in = new FastScanner(is);
        out = new PrintWriter(System.out);
        long s = System.currentTimeMillis();
        read();
        out.flush();
        if (!oj){
            System.out.println("[" + (System.currentTimeMillis() - s) + "ms]");
        }
    }

    public boolean more(){
        return in.hasNext();
    }

    public int ni(){
        return in.nextInt();
    }

    public long nl(){
        return in.nextLong();
    }

    public double nd(){
        return in.nextDouble();
    }

    public String ns(){
        return in.nextString();
    }

    public char nc(){
        return in.nextChar();
    }

    class FastScanner {
        BufferedReader br;
        StringTokenizer st;
        boolean hasNext;

        public FastScanner(InputStream is) throws IOException {
            br = new BufferedReader(new InputStreamReader(is));
            hasNext = true;
        }

        public String nextToken() {
            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (Exception e) {
                    hasNext = false;
                    return "##";
                }
            }
            return st.nextToken();
        }

        String next = null;
        public boolean hasNext(){
            next = nextToken();
            return hasNext;
        }

        public int nextInt() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Integer.parseInt(more);
        }

        public long nextLong() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Long.parseLong(more);
        }

        public double nextDouble() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Double.parseDouble(more);
        }

        public String nextString(){
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return more;
        }

        public char nextChar(){
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return more.charAt(0);
        }
    }

    static class D{

        public static void pp(int[][] board, int row, int col) {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < row; ++i) {
                for (int j = 0; j < col; ++j) {
                    sb.append(board[i][j] + (j + 1 == col ? "\n" : " "));
                }
            }
            System.out.println(sb.toString());
        }

        public static void pp(char[][] board, int row, int col) {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < row; ++i) {
                for (int j = 0; j < col; ++j) {
                    sb.append(board[i][j] + (j + 1 == col ? "\n" : " "));
                }
            }
            System.out.println(sb.toString());
        }
    }

    static class ArrayUtils {

        public static void fill(int[][] f, int value) {
            for (int i = 0; i < f.length; ++i) {
                Arrays.fill(f[i], value);
            }
        }

        public static void fill(int[][][] f, int value) {
            for (int i = 0; i < f.length; ++i) {
                fill(f[i], value);
            }
        }

        public static void fill(int[][][][] f, int value) {
            for (int i = 0; i < f.length; ++i) {
                fill(f[i], value);
            }
        }
    }

    static class Num{
        public static <K> void inc(Map<K, Integer> mem, K k) {
            if (!mem.containsKey(k)) mem.put(k, 0);
            mem.put(k, mem.get(k) + 1);
        }
    }
}

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