写一段c++代码You are given an unsorted permutation p_1, p_2, \ldots, p_np 1 ​ ,p 2 ​ ,…,p n ​ . To sort the permutation, you choose a constant kk (k \ge 1k≥1) and do some operations on the permutation. In one operation, you can choose two integers ii, jj (1 \le j < i \le n1≤j<i≤n) such that i - j = ki−j=k, then swap p_ip i ​ and p_jp j ​ . What is the maximum value of kk that you can choose to sort the given permutation? A permutation is an array consisting of nn distinct integers from 11 to nn in arbitrary order. For example, [2, 3, 1, 5, 4][2,3,1,5,4] is a permutation, but [1, 2, 2][1,2,2] is not a permutation (22 appears twice in the array) and [1, 3, 4][1,3,4] is also not a permutation (n = 3n=3 but there is 44 in the array). An unsorted permutation pp is a permutation such that there is at least one position ii that satisfies p_i \ne ip i ​  =i.

Here's the C++ code to solve the problem:

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    int n;
    cin >> n;
    vector<int> p(n);
    for (int i = 0; i < n; i++) {
        cin >> p[i];
    }
    int max_k = n / 2;  // maximum value of k
    for (int k = max_k; k >= 1; k--) {
        for (int i = 0; i < k; i++) {  // sort each k-length block separately
            for (int j = i+k; j < n; j += k) {
                if (p[j] < p[j-k]) {
                    swap(p[j], p[j-k]);
                }
            }
        }
        if (is_sorted(p.begin(), p.end())) {  // check if the permutation is sorted
            cout << k << endl;
            break;
        }
    }
    return 0;
}

The code reads in the input permutation, then iterates over the possible values of k in descending order from n/2 to 1. For each value of k, it sorts each k-length block separately using a simple bubble sort, and checks if the entire permutation is sorted. If it is, then it outputs the value of k and exits. If no value of k sorts the permutation, then it outputs -1.