Highways（POJ_2485）

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

Huge input,scanf is recommended.

Source

POJ Contest,Author:Mathematica@ZSU

代码

做的第一道最小生成树，具有里程碑式的意义，哈哈~用了并查集，感觉这种做法应该是kruscal吧。。。

#include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;

struct node
{
    int u,v,w;
    bool operator <(const node x)const
    {
        return w<x.w;
    }
} a[30000];

int bin[1100];
int ans[30000],cnt;

int find(int x)
{
    return x==bin[x]?x:find(bin[x]);
}

void merge(int a,int b,int w)
{
    int fa=find(a),fb=find(b);
    if(fa!=fb)
    {
        bin[fa]=fb;
        ans[cnt++]=w;
    }
}

int main()
{
    int t,n;
    cin>>t;
    while(t--)
    {
        cin>>n;
        for(int i=0; i<=n; i++)
            bin[i]=i;
        int k=0,c;
        for(int i=0; i<n; i++)
        {
            for(int j=0; j<n; j++)
            {
                scanf("%d",&c);
                if(i<j)
                {
                    a[k].u=i;
                    a[k].v=j;
                    a[k++].w=c;
                }
            }
        }
        sort(a,a+k);
        cnt=0;
        for(int i=0; i<k; i++)
        {
            merge(a[i].u,a[i].v,a[i].w);
        }
        sort(ans,ans+cnt);
        cout<<ans[cnt-1]<<endl;
    }
    return 0;
}

这种题一般好像都用prim做，附上prim算法的代码

#include <cstdio>
#include <cstring>
#define INF 0x3f3f3f3f
#define N 1001

using namespace std;

int map[N][N];
bool vis[N];
int dis[N];
int n,sum,max;

void prim()
{
    sum=0;
    max=-9999;
    int mm;
    int i,j;
    memset(vis,0,sizeof(vis));
    for(i=0; i<n; i++)
        dis[i]=map[0][i];
    vis[0]=true;
    int pos;
    for(i=0; i<n-1; i++)
    {
        mm=INF;
        for(j=0; j<n; j++)
        {
            if(vis[j]==false&&dis[j]<mm)
            {
                mm=dis[j];
                pos=j;
            }
        }
        sum+=mm;
        if(mm>max) max=mm;
        vis[pos]=true;
        for(j=0; j<n; j++)
        {
            if(vis[j]==false&&dis[j]>map[j][pos])
                dis[j]=map[j][pos];
        }
    }
    printf("%d\n",max);
}

int main()
{
    int i,j,t;
    scanf("%d",&t);
    while(t--)
    {
        scanf("%d",&n);
        for(i=0; i<n; i++)
            for(j=0; j<n; j++)
                scanf("%d",&map[i][j]);
        prim();
    }
    return 0;
}