UVA 796 - Critical Links【求桥】

link:https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=737

题意：
求桥的数目及边，要求输出边的点的次序由小到大

代码：

#include <stdio.h> 
#include <ctime> 
#include <math.h> 
#include <limits.h> 
#include <complex> 
#include <string> 
#include <functional> 
#include <iterator> 
#include <algorithm> 
#include <vector> 
#include <stack> 
#include <queue> 
#include <set> 
#include <map> 
#include <list> 
#include <bitset> 
#include <sstream> 
#include <iomanip> 
#include <fstream> 
#include <iostream> 
#include <ctime> 
#include <cmath> 
#include <cstring> 
#include <cstdio> 
#include <time.h> 
#include <ctype.h> 
#include <string.h> 
#include <assert.h> 
#pragma comment (linker, "/STACK:102400000,102400000")

using namespace std;

int n, m, x, y;

const int MAXN = 1001000;
const int MAXM = 1000100;

int num;
char tmp;
vector<pair<int, int> > a;

struct Edge
{
    int to, next;
    bool cut;//是否为桥的标记
}edge[MAXM];
int head[MAXN], tot;
int Low[MAXN], DFN[MAXN], Stack[MAXN];
int Index, top;
bool Instack[MAXN];
bool cut[MAXN];
int add_block[MAXN];//删除一个点后增加的连通块
int bridge;

void addedge(int u, int v)
{
    edge[tot].to = v; edge[tot].next = head[u]; edge[tot].cut = false;
    head[u] = tot++;
}


void Tarjan(int u, int pre)
{
    int v;
    Low[u] = DFN[u] = ++Index;
    Stack[top++] = u;
    Instack[u] = true;
    int son = 0;
    for (int i = head[u]; i != -1; i = edge[i].next)
    {
        v = edge[i].to;
        if (v == pre)continue;
        if (!DFN[v])
        {
            son++;
            Tarjan(v, u);
            if (Low[u] > Low[v])Low[u] = Low[v];
            //桥
            //一条无向边(u,v)是桥，当且仅当(u,v)为树枝边，且满足DFS(u)<Low(v)。
            if (Low[v] > DFN[u])
            {
                bridge++;
                edge[i].cut = true;
                edge[i ^ 1].cut = true;
                a.push_back(make_pair(min(u, v), max(u, v)));
            }
            //割点
            //一个顶点u是割点，当且仅当满足(1)或(2) (1) u为树根，且u有多于一个子树。
            //(2) u不为树根，且满足存在(u,v)为树枝边(或称父子边，
            //即u为v在搜索树中的父亲)，使得DFS(u)<=Low(v)
            if (u != pre && Low[v] >= DFN[u])//不是树根
            {
                cut[u] = true;
                add_block[u]++;
            }
        }
        else if (Low[u] > DFN[v])
            Low[u] = DFN[v];
    }
    //树根，分支数大于1
    if (u == pre && son > 1)cut[u] = true;
    if (u == pre)add_block[u] = son - 1;
    Instack[u] = false;
    top--;
}

void solve(int N)
{
    memset(DFN, 0, sizeof(DFN));
    memset(Instack, false, sizeof(Instack));
    memset(add_block, 0, sizeof(add_block));
    memset(cut, false, sizeof(cut));
    Index = top = 0;
    bridge = 0;
    for (int i = 1; i <= N; i++)
        if (!DFN[i])
            Tarjan(i, i);
    sort(a.begin(), a.end());
    printf("%d critical links\n", bridge);
    for (int i = 0; i < a.size(); i++)
        printf("%d - %d\n", a[i].first - 1, a[i].second - 1);
    printf("\n");
}

void init()
{
    tot = 0;
    num = 0;
    a.clear();
    memset(head, -1, sizeof(head));
}

int main()
{
    while (~scanf("%d", &n))
    {
        init();
        for (int i = 1; i <= n; i++)
        {
            scanf("%d", &m);
            m += 1;
            getchar();
            scanf("%c", &tmp);
            scanf("%d",&x);
            scanf("%c", &tmp);
            while (x--)
            {
                scanf("%d", &y);
                y += 1;
                addedge(m, y);
                addedge(y, m);
            }
        }
        solve(n);
    }
    return 0;
}