【USACO2.4.3】牛的旅行 最短路径

一定要注意联通两点后，新的连通块的直径并不一定是联通通过这两个点的最长距离，还可能是原来连通块的直径。

因为这个，WA了好久……真心没想到

Compiling...
Compile: OK

Executing...
   Test 1: TEST OK [0.005 secs, 3724 KB]
   Test 2: TEST OK [0.003 secs, 3724 KB]
   Test 3: TEST OK [0.003 secs, 3724 KB]
   Test 4: TEST OK [0.005 secs, 3724 KB]
   Test 5: TEST OK [0.070 secs, 3724 KB]
   Test 6: TEST OK [0.068 secs, 3724 KB]
   Test 7: TEST OK [0.065 secs, 3724 KB]
   Test 8: TEST OK [0.076 secs, 3724 KB]
   Test 9: TEST OK [0.068 secs, 3724 KB]

All tests OK.

#include 
#include 
#include 
#include 
#include  
using namespace std;

int n;
struct node
{
	double x, y; 
	node(int X, int Y)
	{
		x = (double)X;
		y = (double)Y;	
	}
	node(){}
}a[200];

char s[200][200];
double g[200][200];
vectorA,B;

double dis(node A, node B)
{
	return sqrt((A.x - B.x)*(A.x - B.x) + (A.y - B.y)*(A.y - B.y));
}

void init()
{
	cin >> n;
	for (int i = 0; i != n; ++ i)
	{
		int x, y;
		cin >> x >> y;
		a[i] = node(x, y);	
	}
	for (int i = 0; i != n; ++ i)	scanf("%s", s[i]);

//g数组初始化为-1
	for (int i = 0; i != n; ++ i)
		for (int j = 0; j != n; ++ j)	g[i][j] = -1;
//初始化g数组的一些值
	for (int i = 0; i != n; ++ i)
		for (int j =0; j != n; ++ j)
			if (s[i][j] == '1')
				g[i][j] = dis(a[i], a[j]);	
//自己到自己的距离为0
	for (int i = 0; i != n; ++ i)	g[i][i] = 0;

//求最短路
	for (int k = 0; k != n; ++ k)
		for (int i = 0; i != n; ++ i)
			for (int j = 0; j != n; ++ j)
				if (g[i][k] != -1 && g[k][j] != -1)	
				{
					if (g[i][j] == -1)	
					{
						g[i][j] = g[i][k] + g[k][j];	
					}
					else g[i][j] = min(g[i][j], g[i][k] + g[k][j]);	
				}
}

void doit()
{
	double ans = 0x7fffffff;
	double tans = -1;
	for (int i = 0; i != n; ++ i)
		for (int j = 0; j != n; ++ j)	tans = max(tans, g[i][j]);

	for (int i = 0; i != n; ++ i)
		for (int j = 0; j != n; ++ j)
		{
			//i ,j
			if (g[i][j] != -1)	continue;
			double disA = -1, disB = -1;
			for (int k = 0; k != n; ++ k)	disA = max(disA, g[i][k]);
			for (int k = 0; k != n; ++ k)	disB = max(disB, g[j][k]);
			double tmp = disA + disB + dis(a[i], a[j]);
			if (ans > tmp)
			{
				ans = tmp;	
			}
		}
	if (ans < tans)	ans = tans;
	printf("%lf\n", ans);
}

int main()
{
	init();
	doit();	
	return 0;
}