poj1308

简单题

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
using namespace std;

#define maxn 1006

struct Edge
{
    int v, next;
}edge[1000005];

int name[maxn];
int n, ecount, tot, root;
int in[maxn];
int head[maxn];
bool vis[maxn];
bool ok;

void addedge(int a, int b)
{
    edge[ecount].next = head[a];
    edge[ecount].v = b;
    head[a] = ecount;
    in[b]++;
    ecount++;
}

int find(int a)
{
    for (int i = 0; i < n; i++)
        if (name[i] == a)
            return i;
    name[n++] = a;
    return n - 1;
}

void dfs(int root)
{
    vis[root] = true;
    tot++;
    for (int i = head[root]; i != -1; i = edge[i].next)
    {
        if (vis[edge[i].v])
        {
            ok = false;
            return;
        }
        if (!ok)
            return;
        dfs(edge[i].v);
    }
}

int main()
{
    //freopen("t.txt", "r", stdin);
    int t = 0;
    while (1)
    {
        t++;
        memset(in, 0, sizeof(in));
        memset(head, -1, sizeof(head));
        memset(vis, 0, sizeof(vis));
        ecount = 0;
        n = 0;
        tot = 0;
        ok = true;
        int a, b;
        scanf("%d%d", &a, &b);
        if (a < 0 && b < 0)
            return 0;
        if (a == 0 && b == 0)
        {
            printf("Case %d is a tree.\n", t);
            continue;
        }
        a = find(a);
        b = find(b);
        addedge(a, b);
        while (1)
        {
            scanf("%d%d", &a, &b);
            if (a == 0 && b == 0)
                break;
            a = find(a);
            b = find(b);
            addedge(a, b);
        }
        root = -1;
        for (int i = 0; i < n; i++)
            if (in[i] == 0)
                root = i;
        if (root >= 0)
            dfs(root);
        if (ok && tot == n && root >= 0)
            printf("Case %d is a tree.\n", t);
        else
            printf("Case %d is not a tree.\n", t);
    }
    return 0;
}