POJ 2421 Constructing Roads (最小生成树)

题目：

Description

There are N villages, which are numbered from 1 to N, and you should build some roads such that every two villages can connect to each other. We say two village A and B are connected, if and only if there is a road between A and B, or there exists a village C such that there is a road between A and C, and C and B are connected. 

We know that there are already some roads between some villages and your job is the build some roads such that all the villages are connect and the length of all the roads built is minimum.

 

Input

The first line is an integer N (3 <= N <= 100), 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, 1000]) between village i and village j. 
Then there is an integer Q (0 <= Q <= N * (N + 1) / 2). Then come Q lines, each line contains two integers a and b (1 <= a < b <= N), which means the road between village a and village b has been built.

 

Output

You should output a line contains an integer, which is the length of all the roads to be built such that all the villages are connected, and this value is minimum.

 

Sample Input

3
0 990 692
990 0 179
692 179 0
1
1 2

Sample Output

179

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题意：
给出n个点 求最小生成树 其中已经连上了一些边 求需要连上的边的总长

思路：
将已经连上的边的权值标记为0 用prim算法跑最小生成树

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代码：

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <string>
#include <cstring>
#include <algorithm>

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
const int inf=0x3f3f3f3f;
const int maxn=110;
int n,tmp,m,l,r;
int mp[maxn][maxn],dis[maxn],vis[maxn];

int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        dis[i]=inf;
        for(int j=1;j<=n;j++){
            scanf("%d",&mp[i][j]);
        }
    }
    int sum=0;
    scanf("%d",&m);
    for(int i=1;i<=m;i++){
        scanf("%d%d",&l,&r);
        mp[l][r]=0;
        mp[r][l]=0;
    //    vis[l]=1;vis[r]=1;
    //    sum+=mp[l][r];
    }
    for(int i=1;i<=n;i++){
        dis[i]=mp[1][i];
    }
    dis[1]=0;
    vis[1]=1;
    for(int i=1;i<=n;i++){
        tmp=inf;
        int minn=inf;
        for(int j=1;j<=n;j++){
            if(vis[j]==0 && dis[j]<minn){
                tmp=j;
                minn=dis[j];
            }
        }
        if(tmp==inf) break;
        vis[tmp]=1;
        sum+=minn;
        for(int j=1;j<=n;j++){
            if(vis[j]==0 && dis[j]>mp[tmp][j]){
                dis[j]=mp[tmp][j];
            }
        }
    }
    printf("%d
",sum);
    return 0;
}