POJ 2485-Highways 最小生成树Prim算法

题目来源：http://acm.pku.edu.cn/JudgeOnline/problem?id=2485



解题报告：



还是最小生成树问题，Prim算法实现，没用到priority_queue或是set，还是直接用O(n)复杂度来寻找key中的最小值。这道题很基础，纯粹用来熟练Prim算法。

#include <iostream>
using namespace std;

#define INF 70000

int N;//点的数目
int key[500];
bool flag[500];
int A[500][500];

int Prim(int r)
{
	int result=0;
	for(int i=0;i<N;i++)
	{
		key[i]=INF;
		flag[i]=true;
	}
	key[r]=0;
	for(int i=0;i<N;i++)
	{
		int min_num=INF;
		int min;
		for(int j=0;j<N;j++)
		{
			if(flag[j] && min_num>key[j])
			{
				min_num=key[j];
				min=j;
			}
		}
		flag[min]=false;
		if(result<key[min])
			result=key[min];
		for(int j=0;j<N;j++)
		{
			if(flag[j] && A[min][j] < key[j])
				key[j]=A[min][j];
		}
	}
	return result;
}

int main()
{
	int T;
	cin >> T;
	while(T--)
	{
		cin >> N;
		for(int i=0;i<N;i++)
		{
			for(int j=0;j<N;j++)
				scanf("%d",&A[i][j]);
		}	
		cout << Prim(0) << endl;
	}
}

附录：



Highways

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 9754		Accepted: 4580

Description
The island nation of Flatopia is perfectly flat. Unfortunately, Flatopia has no public highways. So the traffic is difficult in Flatopia. The Flatopian government is aware of this problem. They're planning to build some highways so that it will be possible to drive between any pair of towns without leaving the highway system.

Flatopian towns are numbered from 1 to N. Each highway connects exactly two towns. All highways follow straight lines. All highways can be used in both directions. Highways can freely cross each other, but a driver can only switch between highways at a town that is located at the end of both highways.

The Flatopian government wants to minimize the length of the longest highway to be built. However, they want to guarantee that every town is highway-reachable from every other town.
Input
The first line of input is an integer T, which tells how many test cases followed.
The first line of each case is an integer N (3 <= N <= 500), which is the number of villages. Then come N lines, the i-th of which contains N integers, and the j-th of these N integers is the distance (the distance should be an integer within [1, 65536]) between village i and village j. There is an empty line after each test case.
Output
For each test case, you should output a line contains an integer, which is the length of the longest road to be built such that all the villages are connected, and this value is minimum.
Sample Input

1

3
0 990 692
990 0 179
692 179 0

Sample Output

692

Hint