Codeforces 246A Buggy Sorting 【构造】

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本文介绍了一个简单的编程挑战——BuggySorting。任务要求找到一个输入序列，使得提供的排序算法无法正确地将序列按非递减顺序排序。对于特定的输入大小n，文章给出了一个构造反例的方法，并提供了一段实现该解决方案的代码。

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题目链接：Codeforces Buggy Sorting

A. Buggy Sorting
time limit per test2 seconds
memory limit per test256 megabytes
inputstandard input
outputstandard output
Little boy Valera studies an algorithm of sorting an integer array. After studying the theory, he went on to the practical tasks. As a result, he wrote a program that sorts an array of n integers a1, a2, …, an in the non-decreasing order. The pseudocode of the program, written by Valera, is given below. The input of the program gets number n and array a.

loop integer variable i from 1 to n - 1
loop integer variable j from i to n - 1
if (aj > aj + 1), then swap the values of elements aj and aj + 1
But Valera could have made a mistake, because he hasn’t yet fully learned the sorting algorithm. If Valera made a mistake in his program, you need to give a counter-example that makes his program work improperly (that is, the example that makes the program sort the array not in the non-decreasing order). If such example for the given value of n doesn’t exist, print -1.

Input
You’ve got a single integer n (1 ≤ n ≤ 50) — the size of the sorted array.

Output
Print n space-separated integers a1, a2, …, an (1 ≤ ai ≤ 100) — the counter-example, for which Valera’s algorithm won’t work correctly. If the counter-example that meets the described conditions is impossible to give, print -1.

If there are several counter-examples, consisting of n numbers, you are allowed to print any of them.

Examples
input
1
output
-1

题意：让你构造一个序列，使得给定的排序算法不成立。

水题，最后一个取1，前面取2就好了。

AC代码：

#include <iostream>
using namespace std;
int main()
{
    int n; cin >> n;
    if(n == 1 || n == 2) {
        cout << -1 << endl;
    }
    else {
        for(int i = 1; i <= n-1; i++) {
            cout << 2 << ' ';
        }
        cout << 1 << endl;
    }
    return 0;
}