USACO 2.2 Sorting a Three-Valued Sequence 三值的排序

题目大意

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给出一个由1、2、3组成的序列，任意两个数字可以交换，问至少交换几次使序列成为升序。

样例输入&输出

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sample input

9 2 2 1 3 3 3 2 3 1

sample output

4

分析&反思

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代码

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#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;

int n, tot[4], sum[4][4];
int list[1005];

int main() {
	
	freopen("sort3.in", "r", stdin);
	freopen("sort3.out", "w", stdout);
	
	cin >> n;
	for(int i = 1; i <= n; i++) {
		cin >> list[i];
		tot[list[i]]++;
	}
	
	tot[2] += tot[1];
	for(int i = 1; i <= n; i++) {
		if(i <= tot[1]) {
			sum[1][list[i]]++;
			continue;
		}
		if(i <= tot[2]) {
			sum[2][list[i]]++;
			continue;
		}
		{
			sum[3][list[i]]++;
		}
	}
	
	int mins, ans = 0;
	for(int i = 1; i <= 3; i++)
	for(int j = 1; j < i; j++) {
		mins = min(sum[i][j], sum[j][i]);
		sum[i][j] -= mins;
		sum[j][i] -= mins;
		ans += mins;
	}
	
	ans += (sum[1][2] + sum[1][3]) * 2;
	
	cout << ans << endl;
	
	return 0;
}