hdu 1325 Is It A Tree?

#include <iostream>
#include <cstring>
#include <cstdio>

using namespace std;

const int M = 1005;
int vist[M], Set[M];
int du[M];
int a, b, maxn;
int flag, cont, k;
/*
形成树的条件是， 每一个节点的入度只能为1， 
且根只能有一个。
*/

int Find(int x) {

    return x == Set[x]? x : Set[x] = Find(Set[x]);
}

void Union(int x, int y) {

    x = Find(x);
    y = Find(y);
    if(x != y) {

        Set[y] = x;
    }
}

void init() {

    for(int i = 1; i < M; i++) {

        Set[i] = i;
        vist[i] = 0;
        du[i] = 0;
    }
    flag = 1;
    cont = 0;
    k = 0;
    maxn = 0;
}

int main()
{
    int q = 1;
     init();
    while(scanf("%d%d", &a, &b) != EOF) {

        if(a < 0 && b < 0)
            break;
        if(a == 0 && b == 0) {

            for(int i = 1; i < M; i++) {

                if(vist[i] && Set[i] == i) {

                    cont++;
                }
                if(du[i] > 1) {

                    flag = 0;
                    break;
                }

            }
            if(cont >1)
                 flag = 0;
            if(flag)
                 printf("Case %d is a tree.\n", q++);
            else
                printf("Case %d is not a tree.\n", q++);
            flag = 0;
            init();
            continue;
        }
        if(a != b && Find(a) == Find(b)) {

            flag = 0;
        }
        else {

            vist[a] = 1;
            vist[b] = 1;
            du[b]++;
            Union(a, b);
        }

    }
    return 0;
}