Educational Codeforces Round 57 (Rated for Div. 2)（有待更新）

题目链接：http://codeforces.com/contest/1096

第一题：

#include<bits/stdc++.h>
using namespace std;

#define debug puts("YES");
#define rep(x,y,z) for(int (x)=(y);(x)<(z);(x)++)
#define ll long long

#define lrt int l,int r,int rt
#define lson l,mid,rt<<1
#define rson mid+1,r,rt<<1|1
#define root l,r,rt
#define mst(a,b) memset((a),(b),sizeof(a))

#define pii pair<ll,ll>
#define mk(x,y) make_pair(x,y)

const int  maxn =1e5+5;
const int mod=998244353;
const int ub=1e6;
ll powmod(ll x,ll y){ll t; for(t=1;y;y>>=1,x=x*x%mod) if(y&1) t=t*x%mod; return t;}
ll gcd(ll x,ll y){return y?gcd(y,x%y):x;}

int x,y;
int main()
{
    int t;scanf("%d",&t);
    while(t--)
    {
        cin>>x>>y;
        cout<<x<<" "<<y/x*x<<endl;
    }
    return 0;
}

第二题：

#include<bits/stdc++.h>
using namespace std;

#define debug puts("YES");
#define rep(x,y,z) for(int (x)=(y);(x)<(z);(x)++)
#define ll long long

#define lrt int l,int r,int rt
#define lson l,mid,rt<<1
#define rson mid+1,r,rt<<1|1
#define root l,r,rt
#define mst(a,b) memset((a),(b),sizeof(a))

#define pii pair<ll,ll>
#define mk(x,y) make_pair(x,y)

const int  maxn =1e5+5;
const int mod=998244353;
const int ub=1e6;
ll powmod(ll x,ll y){ll t; for(t=1;y;y>>=1,x=x*x%mod) if(y&1) t=t*x%mod; return t;}
ll gcd(ll x,ll y){return y?gcd(y,x%y):x;}

int n;
string s;

int main()
{
    cin>>n>>s;
    int flag=0;
    for(int i=0;i<n-1;i++) if(s[i]!=s[i+1]) flag=1;
    if(flag==0)
    {
        int tmp=n,tmpp=tmp+1;
        if(tmp&1) tmpp>>=1;
        else tmp>>=1;
        cout<<1LL*tmp*tmpp%mod<<endl;
    }
    else{
        if(s[0]==s[n-1])
        {
            int i=0,j=n-1;
            while(s[i+1]==s[i]) i++;
            while(s[j]==s[0]) j--;
            cout<<1LL*(n-j)*(i+2)%mod<<endl;
        }
        else
        {
            int i=0,j=n-1;
            while(s[i+1]==s[i]) i++;
            while(s[j]==s[j-1]) j--;
            int ans=((n-j+1)%mod+mod)%mod+i+1;
            ans=(ans+mod)%mod;
            cout<<ans<<endl;
        }
    }
    return 0;
}

第三题：

#include<bits/stdc++.h>
using namespace std;

#define debug puts("YES");
#define rep(x,y,z) for(int (x)=(y);(x)<(z);(x)++)
#define ll long long

#define lrt int l,int r,int rt
#define lson l,mid,rt<<1
#define rson mid+1,r,rt<<1|1
#define root l,r,rt
#define mst(a,b) memset((a),(b),sizeof(a))

#define pii pair<ll,ll>
#define mk(x,y) make_pair(x,y)

const int  maxn =1e5+5;
const int mod=998244353;
const int ub=1e6;
ll powmod(ll x,ll y){ll t; for(t=1;y;y>>=1,x=x*x%mod) if(y&1) t=t*x%mod; return t;}
ll gcd(ll x,ll y){return y?gcd(y,x%y):x;}

int n,s,x,y;
int d[maxn];

int main()
{
    cin>>n>>s;
    rep(i,1,n)
    {
        cin>>x>>y;
        d[x]++,d[y]++;
    }
    int tmp=0;
    rep(i,1,n+1) if(d[i]==1) tmp++;
    printf("%.9f\n",1.0*s/tmp*2);
    return 0;
}