USACO Section 2.1: Sorting a Three-Valued Sequence

 1 /*
 2 ID: leetcod3
 3 PROG: sort3
 4 LANG: C++
 5 */
 6 #include <iostream>
 7 #include <fstream>
 8 #include <string>
 9 #include <map>
10 #include <vector>
11 #include <set>
12 #include <algorithm>
13 #include <queue>
14 #include <cmath>
15 #include <list>
16 #include <cstring>
17 #include <cstdlib>
18 #include <limits>
19 #include <stack>
20 
21 using namespace std;
22 
23 ofstream fout ("sort3.out");
24 ifstream fin ("sort3.in");
25 
26 int N;
27 
28 int main()
29 {
30     fin >> N;
31     vector<int> input(N);
32     int one, two, three;
33     one = two = three = 0;
34     for (int i = 0; i < N; ++i) {
35         fin >> input[i];
36         if (input[i] == 1) one++;
37         else if (input[i] == 2) two++;
38         else three++;
39     }
40     int pos[4][4] = {0};
41     for (int i = 0; i < one; ++i) pos[1][input[i]]++;
42     for (int i = one; i < one+two; ++i) pos[2][input[i]]++;
43     for (int i = one+two; i < N; ++i) pos[3][input[i]]++;
44     for (int i = 1; i < 4; ++i) {
45         //for (int j = 1; j < 4; ++j) cout << pos[i][j] << " ";
46         //cout << endl;
47     }
48     int ans = 0;
49     for (int i = 1; i <= 3; ++i) {
50         for (int j = i; j <= 3; ++j) {
51             if (i == j) continue;
52             int tmp = min(pos[i][j], pos[j][i]);
53             ans += tmp;
54             pos[i][j] -= tmp;
55             pos[j][i] -= tmp;
56         }
57     }
58     int tmp = 0;
59     //cout << ans << endl;
60     for (int i = 1; i <= 3; ++i) {
61         for (int j = 1; j <= 3; ++j) {
62             if (i == j) continue;
63             tmp += pos[i][j];
64         }
65     }
66     //cout << tmp << endl;
67     ans += tmp / 3 * 2;
68     fout << ans << endl;
69 
70     return 0;
71 }