Telephone network problems

One   Link to original question

Network - POJ 1144 - Virtual Judgehttps://vjudge.net/problem/POJ-1144

Two   Input and output

1  Input

The input contains multiple test cases . Each test case describes a network . The... Of each test case 1 Everything is N. Next up   N  Each row in the row contains a location number , Followed by the number of the place where the place can be reached . Each test case contains only 0 The end of the line .N=0 When the input is over , Don't deal with .

2  Output

For each test case , Output the number of key positions in a single line .

3、 ... and   sample input

5

5 1 2 3 4

0

6

2 1 3

5 4 6 2

0

0

Four   sample output

1

2

5、 ... and   Problem analysis

1  sample input 1, The constructed figure is as follows . In this figure, there are 1 A key point 5.

2  sample input 2, The constructed figure is as follows . There are two key points in this figure , Namely 2 and 5.

This problem is to find the number of cuts , utilize  Tarjan  The algorithm can be solved .

6、 ... and   Code

package graph.poj1144;

import java.util.Scanner;

public class POJ1144 {
    static final int maxn = 1000 + 5;
    static int n;
    static int m;
    static int head[];
    static int cnt;
    static int root;
    static int low[];
    static int dfn[];
    static int cut[];
    static int num;

    static Edge e[] = new Edge[maxn << 1];

    static {
        for (int i = 0; i < e.length; i++) {
            e[i] = new Edge();
        }
    }

    static void add(int u, int v) { //  Add an edge u--v
        e[++cnt].next = head[u];
        e[cnt].to = v;
        head[u] = cnt;
    }

    static void tarjan(int u) { // Please cut point 
        dfn[u] = low[u] = ++num;
        int flag = 0;
        for (int i = head[u]; i != 0; i = e[i].next) {
            int v = e[i].to;
            if (dfn[v] == 0) {
                tarjan(v);
                low[u] = Math.min(low[u], low[v]);
                if (low[v] >= dfn[u]) {
                    flag++;
                    if (u != root || flag > 1) {
                        cut[u] = 1;
                    }
                }
            } else
                low[u] = Math.min(low[u], dfn[v]);
        }
    }

    static void init() {
        head = new int[maxn];
        low = new int[maxn];
        dfn = new int[maxn];
        cut = new int[maxn];
        cnt = num = 0;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        while (true) {
            n = scanner.nextInt();
            if (n == 0)
                break;
            init();
            int u, v;
            while (true) {
                Scanner scanner1 = new Scanner(System.in);
                String nums = scanner1.nextLine();
                String[] s = nums.split(" ");
                u = Integer.parseInt(s[0]);
                if (u == 0) {
                    break;
                }
                for (int i = 1; i < s.length; i++) {
                    v = Integer.parseInt(s[i]);
                    add(u, v);
                    add(v, u);
                }
            }
            for (int i = 1; i <= n; i++) {
                if (dfn[i] == 0) {
                    root = i;
                    tarjan(i);
                }
            }

            int ans = 0;
            for (int i = 1; i <= n; i++)
                if (cut[i] == 1)
                    ans++;
            System.out.println(ans);
        }
    }
}

class Edge {
    int to;
    int next;
}

7、 ... and   test result

Green is the input , White is the output