codeforce 8C Looking for Order

Girl Lena likes it when everything is in order, and looks for order everywhere. Once she was getting ready for the University and noticed that the room was in a mess — all the objects from her handbag were thrown about the room. Of course, she wanted to put them back into her handbag. The problem is that the girl cannot carry more than two objects at a time, and cannot move the handbag. Also, if he has taken an object, she cannot put it anywhere except her handbag — her inherent sense of order does not let her do so.

You are given the coordinates of the handbag and the coordinates of the objects in some Сartesian coordinate system. It is known that the girl covers the distance between any two objects in the time equal to the squared length of the segment between the points of the objects. It is also known that initially the coordinates of the girl and the handbag are the same. You are asked to find such an order of actions, that the girl can put all the objects back into her handbag in a minimum time period.

Input
The first line of the input file contains the handbag’s coordinates xs, ys. The second line contains number n (1 ≤ n ≤ 24) — the amount of objects the girl has. The following n lines contain the objects’ coordinates. All the coordinates do not exceed 100 in absolute value. All the given positions are different. All the numbers are integer.

Output
In the first line output the only number — the minimum time the girl needs to put the objects into her handbag.

In the second line output the possible optimum way for Lena. Each object in the input is described by its index number (from 1 to n), the handbag’s point is described by number 0. The path should start and end in the handbag’s point. If there are several optimal paths, print any of them.

Examples
input
0 0
2
1 1
-1 1
output
8
0 1 2 0
input
1 1
3
4 3
3 4
0 0
output
32
0 1 2 0 3 0

题意

给出n<=24个物品和一个位置(平面直角坐标系)，每次从位置出发，拿1~2个物品再回到位置，问最小距离。

题解

状压，注意剪枝。

#include <cstdio>
#include <cstring>
#define N 25
#define inf 0x33333333
using namespace std;
int f[(1<<24)+10],pre[(1<<24)+10],pr[(1<<24)+10];
int x[N],y[N],dis[N][N],n,ans[N*N],x0,y0;
main()
{
    scanf("%d%d%d",&x0,&y0,&n);
    for (int i=0;i<n;i++)
        scanf("%d%d",&x[i],&y[i]);
    x[n+1]=x0,y[n+1]=y0;
    for (int i=0;i<=n+1;i++)
        for (int j=0;j<=n+1;j++)
            if (i!=j)
                dis[i][j]=(x[i]-x[j])*(x[i]-x[j])+(y[i]-y[j])*(y[i]-y[j]);
    int cnt=(1<<n)-1;
    memset(f,0x33,sizeof f);
    f[0]=0;
    for (int i=0;i<=cnt;i++)
    {
        if (f[i]==inf)
            continue;
        for (int j=0;j<n;j++)
            if (!((1<<j)&i))
            {
                int to=i|(1<<j),val=dis[n+1][j]*2;
                if (f[i]+val<f[to])
                {
                    f[to]=f[i]+val;
                    pre[to]=i;
                    pr[to]=N*N+j+1;
                }
                for (int k=j+1;k<n;k++)
                    if (!(i&(1<<k)))
                    {
                        to=i|(1<<j)|(1<<k);
                        val=dis[n+1][j]+dis[j][k]+dis[n+1][k];
                        if (f[i]+val<f[to])
                        {
                            f[to]=f[i]+val;
                            pre[to]=i;
                            pr[to]=(j+1)*N+k+1;
                        }
                    }
                break;////剪枝
            }
    }
    printf("%d\n",f[cnt]);
    int temp=cnt,tot=0;
    while(temp)
    {
        ans[++tot]=temp;
        temp=pre[temp];
    }
    printf("0 ");
    for (int i=tot;i;i--)
    {
        if (pr[ans[i]]>N*N)
            printf("%d ",pr[ans[i]]-N*N);
        else
            printf("%d %d ",pr[ans[i]]/N,pr[ans[i]]%N);
        printf("0 ");
    }
}