Optimal Number Permutation CodeForces-622 D(法則を探す)

You have array a that contains all integers from 1 to n twice. You can arbitrary permute any numbers in a.
Let number i be in positions xi, yi (xi 
Input The only line contains integer n (1 ≤ n ≤ 5·105).
Output Print 2n integers — the permuted array a that minimizes the value of the sum s.
Example Input 2 Output 1 1 2 2 Input 1 Output 1 1
法則を探る1:1 1 2 : 1 1 2 2 3 : 1 3 1 2 2 3 4 : 1 3 3 1 2 4 2 4 ….

#include

int a[1000100];

int main()
{
    int n;
    scanf("%d",&n);
    a[n*2]=n;

    int end = n*2;
    int tmp = n/2;
    int i,j;

    a[1]=1;
    for(i=2;i<=tmp;i++)
        a[i] = a[i-1]+2;

    if(n%2 && n!=1) a[i] = a[i-1]+2;

    j=tmp+1+n%2;
    if(n%2==0)
        {
            a[j]=a[j-1];
            for(j++; j<=n; j++)
                a[j]=a[j-1]-2;
        }
    else 
        {
            for( j=tmp+1+n%2; j<=n; j++)
                a[j]=a[j-1]-2;
        }


    a[n+1]=2;
    for(i=n+2;i<=n+tmp;i++)
        a[i] = a[i-1]+2;

    j=n+tmp+1;
    if(n%2==0)
        {
            for( ;j<=end-1;j++)
                a[j]=a[j-1]-2;
        }
    else 
        {
            a[j]=a[j-1];
            for(j+=1;j<=end-1;j++)
                a[j]=a[j-1]-2;
        }




    for(i=1;i<=end;i++)
        printf("%d%c",a[i],i==end?'
':' ');

    return 0;
}