好题，自己想没想出来，虽然大家都说是水题
1 /* ID:linyvxi1
2 PROB:sort3
3 LANG:C++
4 */
5 #include <stdio.h>
6 #include <algorithm>
7 using namespace std;
8 int start_pos[1005];
9 int final_pos[1005];
10 int corr_map[4][4]={0};
11 int main()
12 {
13 freopen("sort3.in","r",stdin);
14 freopen("sort3.out","w",stdout);
15
16 int n;
17 int swap_num=0;
18 scanf("%d",&n);
19 int i;
20 for(i=0;i<n;i++){
21 scanf("%d",&start_pos[i]);
22 final_pos[i]=start_pos[i];
23 }
24
25 sort(final_pos,final_pos+n);
26
27 for(i=0;i<n;i++){
28 if(final_pos[i]!=start_pos[i]){
29 swap_num++;
30 corr_map[final_pos[i]][start_pos[i]]++;
31 }
32 }
33 int ans=0;
34 int j;
35 for(i=1;i<=3;i++){
36 for(j=i+1;j<=3;j++){
37 int t=min(corr_map[i][j],corr_map[j][i]);
38 ans+=t;
39 corr_map[i][j]-=t;
40 corr_map[j][i]-=t;
41 swap_num-=2*t;
42 }
43 }
44 ans+=swap_num/3*2;
45 printf("%d\n",ans);
46 }