CodeForces - 233A Perfect Permutation
A. Perfect Permutation
time limit per test: 2 seconds
memory limit per test: 256 megabytes
input: standard input
output: standard output
A permutation is a sequence of integers p1, p2, ..., pn, consisting of n distinct positive integers, each of them doesn't exceed n. Let's denote the i-th element of permutation p as pi. We'll call number n the size of permutation p1, p2, ..., pn.
Nickolas adores permutations. He likes some permutations more than the others. He calls such permutations perfect. A perfect permutation is such permutation p that for any i (1 ≤ i ≤ n) (n is the permutation size) the following equations hold ppi = i and pi ≠ i. Nickolas asks you to print any perfect permutation of size n for the given n.
Input
A single line contains a single integer n (1 ≤ n ≤ 100) — the permutation size.
Output
If a perfect permutation of size n doesn't exist, print a single integer -1. Otherwise print n distinct integers from 1 to n, p1, p2, ..., pn — permutation p, that is perfect. Separate printed numbers by whitespaces.
Examples
input
1
output
-1
input
2
output
2 1
input
4
output
2 1 4 3 - 关键是两个公式：ppi = i and pi ≠ i.；把奇偶两位互换即可，当然n得是偶数；
#include <iostream> using namespace std; int main()
{
int n;
cin >> n;
if( n % 2 == 1 )
{
cout << -1 <<endl;
}
else
{
int i, a[101];
for( i=1; i<=n; i++ )
{
if( i % 2 == 1 )
{
a[i] = i + 1;
}
else
{
a[i] = i - 1;
}
}
for( i=1; i<n; i++ )
{
cout << a[i] << " ";
}
cout << a[i] << endl;
}
return 0;
}
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