HDU 2242（考研路茫茫——空调教室）

题意：有一个教室群，教室之间有空调管相连（原图连通），每个教室都有人，求断开一条空调管使之分开成两个教室群，并且使两个教师群的人数差值最小。

思路：这题没什么难度吧。。思路都很明了（可能是看过标签的缘故吧0.0），求边双连通分量，缩点，然后进行树形dp计数求出答案即可，贴个代码，以后拿来当模板用吧。

#include <vector>
#include <cstdio>
#include <climits>
#include <cstring>
#include <iostream>
#include <algorithm>
#define  pb push_back
#define  sz(x) ((int)(x).size())
using namespace std;

const int N = 10010;
int n, m, head[N], cnt, w[N], dp[N];
struct Edge{
    int e, next, idx;
}edge[N * 4];

void addedge(int a, int b, int idx){
    edge[cnt].e = b;
    edge[cnt].next = head[a];
    edge[cnt].idx = idx;
    head[a] = cnt++;
}

struct tarjan{
    int dfn[N], low[N], times, top, block, belong[N], sta[N * 4];
    void initi(){
        memset(dfn, 0, sizeof dfn);
        memset(belong, -1, sizeof belong);
        times = 1, block = top = 0;
    }
    void dfs(int to, int ed){
        int e;
        dfn[to] = low[to] = times++;
        sta[top++] = to;
        for(int it = head[to];it != -1;it = edge[it].next){
            e = edge[it].e;
            if(edge[it].idx == ed)
                continue;
            if(!dfn[e]){
                dfs(e, edge[it].idx);
                low[to] = min(low[to], low[e]);
            } else
                low[to] = min(low[to], dfn[e]);
        }
        if(dfn[to] == low[to]){
            block++;
            do{
                e = sta[--top];
                belong[e] = block;
                dp[block] += w[e];
            }while(e != to);
        }
    }
}g;

struct DP{
    int vis[N], tot, res;
    vector<int> V[N];
    void initi(){
        memset(dp, 0, sizeof dp);
        memset(vis, 0, sizeof vis);
        memset(w, 0, sizeof w);
        tot = 0, res = INT_MAX;
        for(int i = 1;i < N;i++)
            V[i].clear();
    }
    void dfs(int to){
        vis[to] = 1;
        for(int i = 0;i < sz(V[to]);i++){
            int e = V[to][i];
            if(vis[e])
                continue;
            dfs(e);
            dp[to] += dp[e];
            res = min(res, abs(tot - dp[e] - dp[e]));
        }
    }
}ng;

void solve(){
    g.dfs(0, -1);
    if(g.block == 1){
        printf("impossible\n");
        return ;
    }
    for(int i = 0;i < n;i++){
        for(int it = head[i];it != -1;it = edge[it].next){
            int a = g.belong[i], b = g.belong[edge[it].e];
            if(a != b){
                ng.V[a].pb(b);
                ng.V[b].pb(a);
            }
        }
    }
    ng.dfs(1);
    printf("%d\n", ng.res);
    return ;
}

int main(){
    while(cin >> n >> m){
        int i, a, b;
        memset(head, -1, sizeof head);
        cnt = 0, ng.initi(), g.initi();
        for(i = 0;i < n;i++){
            scanf("%d", &w[i]);
            ng.tot += w[i];
        }
        for(i = 0;i < m;i++){
            scanf("%d%d", &a, &b);
            addedge(a, b, i);
            addedge(b, a, i);
        }
        solve();
    }
	return 0;
}