ural 1416. Confidential (次小生成树模板题)

URAL1416
题意：次小生成树模板题

思路：用Kruskal求最小生成树，标记用过的边。求次小生成树时，依次枚举用过的边，将其去除后再求最小生成树，得出所有情况下的最小的生成树就是次小的生成树。复杂度o(m2)。。。貌似有其他优化。。。

 

写的时候。。因为点数是500。。我把边集的数组大小开成了500.。。交了10遍越界才意识到问题在哪里。。。真的是智商掉线啊orz…

C++
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/* ***********************************************
Author :111qqz
Created Time :2016年07月11日 星期一 20时44分28秒
File Name :code/ural/1416.cpp
************************************************ */
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <cmath>
#include <cstdlib>
#include <ctime>
#define fst first
#define sec second
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define ms(a,x) memset(a,x,sizeof(a))
typedef long long LL;
#define pi pair < int ,int >
#define MP make_pair
using namespace std;
const double eps = 1E-8;
const int dx4[4]={1,0,0,-1};
const int dy4[4]={0,-1,1,0};
const int inf = 0x3f3f3f3f;
const int N=505;
int n,m;
int f[N];
int mst;
int ans;
int cnt = 0 ;
vector < pi > e[N*N];
bool flag;
struct Edge
{
    int u,v;
    int w;
    int in;//标记边是否在生成树中。
    bool operator < (Edge b)const
    {
        return w<b.w;
    }
    void input()
    {
        scanf("%d%d%d",&u,&v,&w);
    }
}E[N*N];
void init()
{
    for ( int i = 1 ; i <= n ; i++) f[i] = i;
    for ( int i = 1 ; i <= n ; i++) e[i].clear();
}
int root ( int x)
{
  //  cout<<"x:"<<x<<" f[x]:"<<f[x]<<endl;
    if (x!=f[x]) f[x] = root (f[x]);
    return f[x];
}
void merge( int x,int y)
{
    int rx = root (x);
    int ry = root (y);
    if (rx==ry) return ;
    f[rx] = ry;
  //  if (rx<ry) f[ry] = rx;
  //  else f[rx]=ry;
}
void kruskal(int k)
{
    for ( int i = 1 ; i <= n ; i++) f[i] = i ;
    mst = 0 ;
    cnt =  0;
    for ( int i = 1; i <= m ; i++)
    {
        int u = E[i].u;
        int v = E[i].v;
        int w = E[i].w;
        if (i==k) continue;
        if (root(u)==root(v)) continue;
        merge(u,v);
        cnt++;
        mst+=w;
        if (cnt>=n-1) break;
    }
    if (cnt<n-1) return ;
    ans = min(ans,mst);
}
int main()
{
        #ifndef  ONLINE_JUDGE
        freopen("code/in.txt","r",stdin);
  #endif
        cin>>n>>m;
        init();
        for ( int i = 1 ; i <= m ; i++)
        {
            E[i].input();
            int u = E[i].u;
            int v = E[i].v;
            int w = E[i].w;
 
            e[u].push_back(make_pair(v,w));
            e[v].push_back(make_pair(u,w));
        }
        sort(E+1,E+m+1);
        mst = 0 ;
        cnt = 0 ;
        for ( int i = 1 ; i <= m ; i++)
        {
            int u = E[i].u;
            int v = E[i].v;
            int w = E[i].w;
            if (root(u)==root(v)) continue;
            E[i].in = 1;
            merge(u,v);
            cnt++;
            mst+=w;
        }
        if (cnt<n-1)
        {
            puts("Cost: -1");
            puts("Cost: -1");
            return 0;
        }
        printf("Cost: %d\n",mst);
        ans = inf;
        for ( int i = 1 ; i <= m ; i++)
        {
            if (E[i].in==0) continue;
           // cout<<"hhh?"<<endl;
            kruskal(i);
        }
        if (ans==inf) ans = -1;
        printf("Cost: %d\n",ans);
  #ifndef ONLINE_JUDGE  
  fclose(stdin);
  #endif
    return 0;
}
 
 
 
 

 

作者： CrazyKK

ex-ACMer@hust，stackoverflow-engineer@sensetime

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