Freckles

3
1.0 1.0
2.0 2.0
2.0 4.0

3.41

英文不好的话，阅读本题实在有些难度，里面出现的关键词如freckles等都是六级词汇。呵呵了......下面解释一下该题的意思，大意就是在二维坐标系内，知道一些点的坐标，用线将这些点连通，保持线的总长度最短。哈哈这还是最小生成树问题啊，换汤不换药啊！好，不多说，上菜了！

#include <iostream>
#include <algorithm>
#include <iomanip>

using namespace std;
int visited[101];

struct Edge{
    int vertex1;
    int vertex2;
    double cost;
};

struct Point{
    double x;
    double y;
    double computeDistance(Point A)
    {
        double tmp = (x - A.x)*(x - A.x) + (y - A.y)*(y - A.y);
        return sqrt(tmp);
    }
}points[101];
int cmp(const void *a, const void *b)
{
    Edge *e1 = (Edge*) a;
    Edge *e2 = (Edge*) b;
    return e1->cost - e2->cost;
}

Edge edges[5000];
int main()
{
    int n;
    double costs;
    while (cin >> n && n != 0)
    {
        int size = 0;
        costs = 0;
        for (int i = 0; i <= n; i++)
            visited[i] = 0;
        for (int i = 1; i <= n; i++)
        {
            cin >> points[i].x >> points[i].y;
        }
        for (int i = 1; i <= n; i++)
        {
            for (int j = i + 1; j <= n; j++)
            {
                edges[size].vertex1 = i;
                edges[size].vertex2 = j;
                edges[size].cost = points[i].computeDistance(points[j]);
                size++;
            }
        }

        qsort(edges, size, sizeof(Edge), cmp);

        for (int j = 0; j < size; j++)
        {
            if (visited[edges[j].vertex1] == 0 || visited[edges[j].vertex2] == 0)
            {
                costs += edges[j].cost;
                visited[edges[j].vertex1] = 1;
                visited[edges[j].vertex2] = 1;
            }
        }
        cout <<fixed << setprecision(2);
        cout << costs << endl;
    }

    return 0;
}

关于用cout控制小数位数的输出，可以参考前面的文章http://www.cnblogs.com/tgycoder/p/5010463.html（C++输入输出流格式控制）

再给个例子回顾一下：

#include <iostream>
#include <iomanip>

using namespace std;

int main(void)

{

    const double value = 12.3456789;

    cout << value << endl; // 默认以6精度，所以输出为 12.3457

    cout << setprecision(4) << value << endl; // 改成4精度，所以输出为12.35

    cout << setprecision(8) << value << endl; // 改成8精度，所以输出为12.345679

    cout << fixed << setprecision(4) << value << endl; // 加了fixed意味着是固定点方式显示，所以这里的精度指的是小数位，输出为12.3457

    cout << value << endl; // fixed和setprecision的作用还在，依然显示12.3457

    cout.unsetf(ios::fixed); // 去掉了fixed，所以精度恢复成整个数值的有效位数，显示为12.35

    cout << value << endl;

    cout.precision(6); // 恢复成原来的样子，输出为12.3457

    cout << value << endl;
}