UVALive 3523 Knight of Round Table 圆桌骑士 (无向图点双连通分量)

#include<bits/stdc++.h>
using namespace std;
#define bug(x) cout<<#x<<‘=‘<<x<<endl;
const int maxv = 1000+5;
const int maxe = maxv*maxv;//开小

int dfn[maxv],low[maxv],bccno[maxv],dfs_clock,bcc_cnt;
bool iscut[maxv];
vector<int> bcc[maxv];
int head[maxv],fro[maxe],to[maxe],nxt[maxe],ecnt;

int edges[maxe],top;

void addEdge(int u,int v)
{
    fro[ecnt] = u;
    to[ecnt] = v;
    nxt[ecnt] = head[u];
    head[u] = ecnt++;
}

void tarjan(int u,int fa)
{
    dfn[u] = low[u] = ++dfs_clock;
    for(int i = head[u]; ~i; i = nxt[i]){
        int v = to[i];

        if(!dfn[v]){
            edges[++top] = i;
            tarjan(v,i);
            low[u] = min(low[v],low[u]);
            if(low[u] >= dfn[u]){
                iscut[u] = true;
                bcc_cnt++; bcc[bcc_cnt].clear(); //bcc从1开始
                int U,V;
                do{
                    int e = edges[top--];
                    U = fro[e], V = to[e];
                    if(bccno[U] != bcc_cnt) { bcc[bcc_cnt].push_back(U); bccno[U]=bcc_cnt; }
                    if(bccno[V] != bcc_cnt) { bcc[bcc_cnt].push_back(V); bccno[V]=bcc_cnt; }
                }while(U!=u||V!=v);
            }
        }else  if((i^1) != fa && dfn[v] < dfn[u] ){ low[u] = min(low[u],dfn[v]);  edges[++top] = i; }// == 比 ^优先级高！第二个条件少了会WA是什么原因
    }
}

void find_bcc(int n)
{
    top = 0;
    memset(dfn ,0,sizeof(dfn));
    memset(iscut, 0,sizeof(iscut));
    memset(bccno, 0,sizeof(bccno));
    dfs_clock = bcc_cnt = 0;
    for(int i = 0; i <  n; i++) {
        if(!dfn[i]) {
            tarjan(i,-1);
            iscut[i] = ~nxt[head[i]];// !(nxt[head[i]] == -1);//树根特判,如果只有一个子结点false
        }
    }
}

int color[maxv];
bool bipartite(int u, int b)
{
    for(int i = head[u]; ~i; i = nxt[i]){
        int v = to[i]; if(bccno[v] != b) continue;
        if(color[v] == color[u]) return false;
        if(!color[v]){
            color[v] = 3 - color[u];
            if(!bipartite(v,b)) return false;
        }
    }
    return true;
}

bool hate[maxv][maxv];
bool inOdd[maxv];

int main()
{
    //freopen("in.txt","r",stdin);
    int n,m;
    while(~scanf("%d%d",&n,&m)&&(n||m)){
        memset(hate,0,sizeof(hate));
        while(m--){
            int u,v; scanf("%d%d",&u,&v); u--,v--;
            hate[u][v] = hate[v][u] = true;
        }
        memset(head,-1,sizeof(head)); ecnt = 0;
        for(int i = 0; i < n; i++)
        for(int j = i+1; j < n; j++) if(!hate[i][j]){
            addEdge(i,j); addEdge(j,i);
        }
        find_bcc(n);
       // bug(bcc_cnt);
        memset(inOdd,0,sizeof(inOdd));
        for(int i = 1; i <= bcc_cnt; i++){
            memset(color,0,sizeof(color));
            for(int j = 0; j < bcc[i].size(); j++) bccno[bcc[i][j]] = i; //割点，属于多个连通分量，因此特殊处理
            int u = bcc[i][0];
            color[u] = 1;
            if(!bipartite(u,i)){
                for(int j = 0; j < bcc[i].size(); j++) inOdd[bcc[i][j]] = true;
            }
        }
        int ans = n;
        for(int i= 0; i < n; i++) if(inOdd[i]) ans--;
        printf("%d\n",ans);
    }
    return 0;
}