【Codeforces 986B】Petr and Permutations

【链接】 我是链接,点我呀:)
【题意】

题意

【题解】

n为奇数时3n和7n+1奇偶性不同
n为偶数时也是如此
然后交换任意一对数
逆序对的对数的奇偶性会发生改变一次
求出逆序对 对n讨论得出答案。

【代码】

import java.io.*;
import java.util.*;

public class Main {
    
    static InputReader in;
    static PrintWriter out;
        
    public static void main(String[] args) throws IOException{
        //InputStream ins = new FileInputStream("E:\\rush.txt");
        InputStream ins = System.in;
        in = new InputReader(ins);
        out = new PrintWriter(System.out);
        //code start from here
        new Task().solve(in, out);
        out.close();
    }
    
    static int N = (int)1e6;
    static class Task{
        int n;
        int a[] = new int[N+10];
        int temp[] = new int[N+10];
        int ans = 0;
        
        public void mergesort(int l,int r) {
            if (l>=r) return;
            int mid = (l+r)/2;
            mergesort(l,mid);mergesort(mid+1,r);
            int k = l,i = l,j = mid+1;
            while (i <= mid && j <= r) {
                if (a[i]>a[j]) {
                    ans = ans + mid-i+1;
                    ans = ans % 2;
                    temp[k++] = a[j];
                    j++;
                }else {
                    temp[k++] = a[i];
                    i++;
                }
            }
            while (i<=mid) {
                temp[k++] = a[i];
                i++;
            }
            while (j<=r) {
                temp[k++] = a[j];
                j++;
            }
            for (i = l;i <= r;i++) a[i] = temp[i];
            
        }
        
        public void solve(InputReader in,PrintWriter out) {
            n = in.nextInt();
            for (int i = 1;i <= n;i++) {
                a[i] = in.nextInt();
            }
            mergesort(1,n);
            if (n%2==1) {
                if (ans==1) {
                    out.println("Petr");
                }else {
                    out.println("Um_nik");
                }
            }else {
                if (ans==1) {
                    out.println("Um_nik");
                }else {
                    out.println("Petr");
                }
            }
        }
    }

    

    static class InputReader{
        public BufferedReader br;
        public StringTokenizer tokenizer;
        
        public InputReader(InputStream ins) {
            br = new BufferedReader(new InputStreamReader(ins));
            tokenizer = null;
        }
        
        public String next(){
            while (tokenizer==null || !tokenizer.hasMoreTokens()) {
                try {
                tokenizer = new StringTokenizer(br.readLine());
                }catch(IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }
        
        public int nextInt() {
            return Integer.parseInt(next());
        }
    }
}