Codeforces Round #579 (Div. 3)

Posted 2022-07-14 the-pines-of-star

A - Circle of Students

题意：判断是否是1,2,3...n的排列或者n,n-1...2,1的排列

思路1：分类讨论，如果是递增的，ai-i的值固定，如果递减，ai+i的值固定

#include<bits/stdc++.h>
#define rep(i,a,b) for(int i=a;i<=b;i++)
#define dep(i,a,b) for(int i=b;i>=a;i--)
using namespace std;
#define ll long long
const int N=3e5+5;
const int mod = 998244353;
int T,n,a[101010],flag;
ll rd()

    ll x=0,f=1;char ch=getchar();
    while(ch<‘0‘||ch>‘9‘)if(ch==‘-‘)f=-1;ch=getchar();
    while(ch>=‘0‘&&ch<=‘9‘)x=x*10+ch-‘0‘;ch=getchar();
    return x*f;

 
int main()

    T=rd();
    while(T--)
    
        n=rd();
        flag=1;
        rep(i,1,n) a[i]=rd();
        rep(i,2,n) 
        
            if((a[i]-a[1]+n)%n!=(i-1))    flag=0;
            
        if(flag) 
        
            printf("YES\n");
            continue;
        
        flag=1;
        rep(i,2,n) 
        
            if((a[i]+i)%n!=(a[1]+1)%n)    flag=0;
            
        if(!flag) printf("NO\n");
        else printf("YES\n");
     
 

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思路2：直接做

#include<bits/stdc++.h>
#define rep(i,a,b) for(int i=a;i<=b;i++)
#define dep(i,a,b) for(int i=b;i>=a;i--)
using namespace std;
#define ll long long
const int N=3e5+5;
const int mod = 998244353;
int T,n,a[101010],flag;
ll rd()

    ll x=0,f=1;char ch=getchar();
    while(ch<‘0‘||ch>‘9‘)if(ch==‘-‘)f=-1;ch=getchar();
    while(ch>=‘0‘&&ch<=‘9‘)x=x*10+ch-‘0‘;ch=getchar();
    return x*f;

 
int main()

    T=rd();
    while(T--)
    
        n=rd();
        flag=1;
        rep(i,1,n) a[i]=rd();
        rep(i,2,n) 
        
            if(a[i]-1==a[i-1]) continue;
            else if(a[i]==1&& a[i-1]==n) continue;
            flag=0;
        
        if(flag) 
        
            printf("YES\n");
            continue;
        
        flag=1;
        rep(i,2,n) 
        
            if(a[i]+1==a[i-1]) continue;
            else if(a[i]==n&& a[i-1]==1) continue;
            flag=0;
        
        if(!flag) printf("NO\n");
        else printf("YES\n");
     
 

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