Codeforces Round #411 (Div. 2)（四道大水题）

​​Codeforces Round #411 (Div. 2)​​​
四道大水题，写完四道就凉 了。

​​A - Fake NP CodeForces - 805A​​

#include<iostream>
#include<cstdio>
using namespace std;
int main()
{
    int l ,r;
    scanf("%d%d",&l,&r);
    if(l==r) printf("%d\n",l);
    else
    printf("2\n");
    return 0;
}

​​B - 3-palindrome CodeForces - 805​​

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int main()
{
    int n;
    scanf("%d",&n);
    if(n==1) printf("a\n");
    else if(n==2) printf("aa\n");
    else if(n==3) printf("bba\n");
    else if(n==4) printf("bbaa\n");
    else//
    {
        string s="bbaa";
        for(int i=4;i<n;i++)
            s+=s[i-2]=='a'?'b':'a';
        cout<<s<<endl;
    }
    return 0;
}

​​C - Find Amir CodeForces - 805C​​

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int main()
{
    int n;//i+j
    scanf("%d",&n);
    if(n<=2) printf("0\n");
    else
    {
        printf("%d\n",(n-1)/2);
    }
    return 0;
}

​​D - Minimum number of steps CodeForces - 805D​​

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
typedef long long ll;
const ll mod=1e9+7;//
int main()
{
    string s;
    cin>>s;
    int len=s.size();
    ll cnt=0;
    ll ans=0;
    for(int i=len-1;i>=0;i--)
    {
        if(s[i]=='b') cnt++;
        else if(s[i]=='a')
        {
            ans=(ans+cnt)%mod;
            cnt=(cnt*2)%mod;
        }
    }
    printf("%lld\n",ans);

    return 0;
}