BestCoder Round #53 (div.2)(hdu5422，hdu5423，hdu5424)

1.Rikka with Graph

题目链接：

http://acm.hdu.edu.cn/showproblem.php?pid=5422

解题思路：

如果连上1-$n$的边，最短距离就是1。所以所有情况下最短距离都是1。

考虑方案数，如果本来没有1-$n$的边，那么只能连1-$n$，方案数为1。否则怎么连都可以，方案数是\frac{n(n-1)}{2}​2​​n(n−1)​​。

AC代码：

#include <iostream>#include <cstdio>using namespace std;int main(){    int n,m;    while(~scanf("%d%d",&n,&m)){        int u,v;        int flag = 0;        for(int i = 0; i < m; i++){            scanf("%d%d",&u,&v);            if(u == 1 && v == n || u == n && v == 1)                flag = 1;        }        if(flag)            printf("1 %d\n",n*(n-1)/2);        else            printf("1 1\n");    }    return 0;}

2.Rikka with Tree

题目链接：

http://acm.hdu.edu.cn/showproblem.php?pid=5423

解题思路：

http://blog.csdn.net/piaocoder/article/details/48129539

AC代码：

#include <iostream>  #include <cstdio>  #include <cstring>  #include <vector>  using namespace std;    const int N = 1005;  vector<int> v[N];  int sum[N];  int cnt;//深度    void dfs(int u,int fa,int dep){      int l = v[u].size();      for(int i = 0; i < l; i++){          int x = v[u][i];          if(x == fa)              continue;          dfs(x,u,dep+1);      }      sum[dep]++;      cnt = max(cnt,dep);  }    bool check(){      for(int i = 1; i < cnt; i++)          if(sum[i] > 1)              return false;      return true;  }    int main(){      int n;      while(~scanf("%d",&n)){          int x,y;          for(int i = 1; i<=n; i++)              v[i].clear();          for (int i = 1; i<n; i++){              scanf("%d%d",&x,&y);              v[x].push_back(y);              v[y].push_back(x);          }          memset(sum,0,sizeof(sum));          cnt = 0;          dfs(1, 1, 1);          if(check())              printf("YES\n");          else              printf("NO\n");      }      return 0;  }  

3.Rikka with Graph II

题目链接：

http://acm.hdu.edu.cn/showproblem.php?pid=5424

解题思路：

http://blog.csdn.net/piaocoder/article/details/48129721

AC代码：

#include <iostream>#include <cstdio>#include <cstring>using namespace std;int n,cnt;int deg[1005],Map[1005][1005];bool flag,vis[1005];void dfs(int u){    if(cnt == n){        flag = 1;        return;    }    for(int i = 1; i <=n && !flag; i++)        if(!vis[i] && Map[u][i])        {            vis[i] = 1;            cnt++;            dfs(i);            cnt--;            vis[i] = 0;        }}int main(){    while(~scanf("%d",&n)){        flag=0;        cnt=1;        memset(deg,0,sizeof(deg));        memset(vis,0,sizeof(vis));        memset(Map,0,sizeof(Map));        int x,y;        for(int i = 0; i < n; i++){            scanf("%d%d",&x,&y);            if(x != y && !Map[x][y]){                Map[x][y]=Map[y][x]=1;                deg[x]++;                deg[y]++;            }        }        int s = 1,tot = 0;        for(int i = 1; i <= n; i++)            if(deg[i] == 1){                s = i;                tot++;        }        if(tot > 2){            printf("NO\n");            continue;        }        vis[s] = 1;        dfs(s);        if(flag)            printf("YES\n");        else            printf("NO\n");    }    return 0;}