hdu 1272

并查集，这题打代码只用了半个小时，调试却调了四个小时，一直WA，最后发现竟是当输入的图一个顶点也没有的时候，必须输出Yes……无语啊，这种题目真是没意思，特殊情况都不说明，害死人！

/*
 * hdu1272/linux.c
 * Created on: 2011-8-4
 * Author    : ben
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

#define MAXN 100005

int parent[MAXN];
int exist[MAXN];

int myfind(int i) {
    int r = i;
    while (parent[r] != r) {
        r = parent[r];
    }
    return r;
}

void merge(int a, int b) {
    if (a < b) {
        parent[b] = a;
    } else {
        parent[a] = b;
    }
}

void work() {
    int a, b, flag, edge, vertex, i;
    scanf("%d%d", &a, &b);
    while (a != -1 && b != -1) {
        if(a == 0 && b == 0) {
            puts("Yes");
            scanf("%d%d", &a, &b);
            continue;
        }
        flag = 1;
        edge = vertex = 0;
        memset(exist, 0, sizeof(exist));
        for (i = 0; i < MAXN; i++) {
            parent[i] = i;
        }
        while (a != 0 && b != 0) {
            if (!exist[a]) {
                exist[a] = 1;
                vertex++;
            }
            if (!exist[b]) {
                exist[b] = 1;
                vertex++;
            }
            edge++;
            if (myfind(a) == myfind(b)) {
                flag = 0;
            } else {
                merge(myfind(a), myfind(b));
            }
            scanf("%d%d", &a, &b);
        }
        if (flag && (vertex == edge + 1)) {
            puts("Yes");
        } else {
            puts("No");
        }
        scanf("%d%d", &a, &b);
    }
}

int main() {
#ifndef ONLINE_JUDGE
    freopen("data.in", "r", stdin);
#endif
    work();
    return 0;
}