HDOJ2242解题报告【边双连通分量】

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <vector>
#include <ctime>
#include <map>
#include <assert.h>
#include <stack>
#include <set>
#include <queue>
#include <cstring>
#include <algorithm>
using namespace std;

#define sacnf scanf
#define scnaf scanf
#define pb push_back
#define Len size()
#define maxn 10010
#define maxm 20010
#define inf 1061109567
//const long long inf=1e15;
#define INF 0x3f3f3f3f
#define Eps 0.000001
const double PI=acos(-1.0);
#define mod 1000000007
#define MAXNUM 10000
#define For(i,j,k) for(int (i)=(j);(i)<=(k);(i)++)
#define mes(a,b) memset((a),(b),sizeof(a))
#define scanfi(a) scanf("%d",&(a))
typedef long long ll;
typedef unsigned long long ulld;
void Swap(int &a,int &b) {int t=a;a=b;b=t;}
ll Abs(ll x) {return (x<0)?-x:x;}

struct Edge
{
    int from, to, next;
} edge[maxm];

int head[maxn],edgenum;
int low[maxn], dfn[maxn],dfs_clock;
int ebcno[maxn], ebc_cnt;
stack<int> S;
bool Instack[maxn];
vector<int> ebc[maxn],G[maxn];
int total,ans;
int num1[maxn],num2[maxn],d[maxn];


void addEdge(int u,int v)
{
    edge[edgenum].from=u;edge[edgenum].to=v;
    edge[edgenum].next=head[u];head[u]=edgenum++;
}

void tarjan(int u,int fa)
{
    int flag=0;
    low[u]=dfn[u]=++dfs_clock;
    S.push(u);Instack[u]=true;
    for(int i=head[u];i!=-1;i=edge[i].next)
    {
        int v=edge[i].to;
        if(v==fa&&!flag) {flag=1;continue;}
        if(!dfn[v])
        {
            tarjan(v,u);
            low[u]=min(low[u],low[v]);
        }
        else if(Instack[v]) low[u]=min(low[u],dfn[v]);
    }
    if(low[u]==dfn[u])
    {
        ebc_cnt++;
        ebc[ebc_cnt].clear();
        for(;;)
        {
            int v=S.top();S.pop();Instack[v]=false;
            ebcno[v]=ebc_cnt;ebc[ebc_cnt].push_back(v);
            if(v==u) break;
        }
    }
}

void find_cut(int l, int r)
{
    dfs_clock=ebc_cnt=0;
    mes(low,0);mes(dfn,0);mes(ebcno,0);
    mes(Instack,false);
    For(i,l,r)
        if(!dfn[i]) tarjan(i,-1);
}

void dfs(int u, int fa)
{
     d[u]=num2[u];int len=G[u].Len;
     For(i,0,len-1)
     {
         int v=G[u][i];
         if(v==fa) continue;
         dfs(v,u);d[u]+=d[v];
     }
     ans=min(ans,abs(total-2*d[u]));
}

int main()
{
    //freopen("data.in","r",stdin);
    //freopen("data.out","w",stdout);
    //clock_t st=clock();
    int n,m;
    while(~scanf("%d%d",&n,&m))
    {
        mes(head,-1);edgenum=0;total=0;
        For(i,1,n) scanfi(num1[i]),total+=num1[i];
        int a,b;
        For(i,1,m)
        {
            scanf("%d%d", &a,&b);a++;b++;
            addEdge(a,b);addEdge(b,a);
        }
        find_cut(1,n);
        if(ebc_cnt==1)
        {
            printf("impossible\n");
            continue;
        }
        For(i,1,ebc_cnt)
        {
            G[i].clear();int sum=0,len=ebc[i].Len;
            For(j,0,len-1) sum+=num1[ebc[i][j]];
            num2[i]=sum;
        }
        for(int i=0;i<edgenum;i+=2)
        {
            int u=ebcno[edge[i].from];
            int v=ebcno[edge[i].to];
            if(u!=v)
                G[u].push_back(v),G[v].push_back(u);
        }
        ans=inf;dfs(1,-1);
        printf("%d\n",ans);
    }
    //clock_t ed=clock();
    //printf("\n\nTime Used : %.5lf Ms.\n",(double)(ed-st)/CLOCKS_PER_SEC);
    return 0;
}