【算法练习】HihoCoder 1183 连通性一·割边与割点

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题意

给出无向图，求出图中的割边与割点。

题解

Tarjan算法。

代码

#include<bits/stdc++.h>
using namespace std;
typedef double db;
typedef long long ll;
typedef unsigned long long ull;
const int nmax = 1e6+7;
const int INF = 0x3f3f3f3f;
const ll LINF = 0x3f3f3f3f3f3f3f3f;
const ull p = 67;
const ull MOD = 1610612741;
int n, m;
int head[nmax], tot, dfs_clock;
int dfn[nmax], low[nmax];
bool iscut[nmax];
struct edge{
    int to, nxt;
}e[nmax<<1];
void add_edge(int u, int v) {
    e[tot].to = v;
    e[tot].nxt = head[u];
    head[u] = tot ++;
}
typedef pair<int, int> pii;
bool cmp(pii a, pii b) {
    if(a.first == b.first)
        return a.second < b.second;
    else
        return a.first < b.first;
}
vector<int> cp;
vector<pair<int, int> > cedge;

void get_cut(int u, int fa) {
    dfn[u] = low[u] = ++dfs_clock;
    int child = 0;
    for(int i = head[u]; i != -1; i = e[i].nxt) {
        int v = e[i].to;
        if(!dfn[v]) {
            child ++;
//            printf("debug %d %d\n", u, v);
            get_cut(v, u);
            low[u] = min(low[u], low[v]);
            if(low[v] >= dfn[u]) {
                if(!iscut[u] && fa != -1) {
                    cp.push_back(u);
                    iscut[u] = true;
                }
            }
            if(low[v] > dfn[u]) {
                if(u < v)
                    cedge.push_back(make_pair(u,v));
                else
                    cedge.push_back(make_pair(v,u));
            }
        } else if(v != fa) {
            low[u] = min(low[u], dfn[v]);
        }
    }
    if(fa == -1 && child > 1) {
        iscut[u] = true;
        cp.push_back(u);
    }

}
int main(){
    memset(head, -1, sizeof head);
    scanf("%d %d", &n, &m);
    for(int i = 1; i <= m; ++i) {
        int u, v;
        scanf("%d %d", &u, &v);
        add_edge(u, v);
        add_edge(v, u);
    }

    for(int i = 1; i <= n; ++i) {
        if(!dfn[i])
            get_cut(i,-1);
    }
//    for(int i = 1; i <= n; ++i) {
//        printf("%d ", dfn[i]);
//    }
//    printf("\n");
//    for(int i = 1; i <= n; ++i) {
//        printf("%d ", low[i]) ;
//    }
//    printf("\n");
    sort(cp.begin(), cp.end());
    sort(cedge.begin(), cedge.end(), cmp);
    if(cp.size() == 0) {
        printf("Null\n");
    } else {
        for(int i = 0; i < cp.size(); ++i) {
            printf("%d ",cp[i]);
        }
        printf("\n");
    }
    for(int i = 0; i < cedge.size(); ++i) {
        printf("%d %d\n",cedge[i].first, cedge[i].second);
    }

    return 0;
}