HDU [P3849]

tarjan 求 无向图的割边 (桥)

边 (x,y) 是桥当且仅当, 对于 x 的子节点 y ,low[x] < dfn[y]
对于连向父节点的边要特殊处理

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <string>
#pragma comment(linker, "/STACK:16777216")
#include <map>
using namespace std;
const int MAXN = 10005, MAXM = 200005;
int head[MAXN], nume, n, m, dfn[MAXN], low[MAXN], ind, cnt, T, tot;
map <string, int> Hash;
string s[MAXN];
bool f[MAXM];
struct edge{
	int to, nxt;
}e[MAXM];
void adde(int from, int to) {
	e[++nume].to = to;
	e[nume].nxt = head[from];
	head[from] = nume;
}
void tarjan(int u, int id) {
	dfn[u] = low[u] = ++ind;
	for(int i = head[u]; i; i = e[i].nxt) {
		int v = e[i].to;
		if(!dfn[v]) {
			tarjan(v, i);
			low[u] = min(low[u], low[v]);
			if(low[v] > dfn[u]) {
				f[i] = f[((i - 1) ^ 1) + 1] = 1;
			}
		}else if(i != ((id - 1) ^ 1) + 1) low[u] = min(low[u], dfn[v]);
	}
}
int main() {
	cin >> T;
	while(T--) {
		cin >> n >> m;
		memset(head, 0, sizeof(head));
		memset(f, 0, sizeof(f));
		nume = ind = tot = cnt = 0;
		memset(dfn, 0, sizeof(dfn));
		memset(low, 0, sizeof(low));
		Hash.clear();
		for(int i = 1; i <= m; i++) {
			string s1, s2;
			cin >> s1 >> s2;
			int u = Hash[s1];
			if(!u) u = ++tot, s[u] = s1;
			Hash[s1] = u;
			int v = Hash[s2];
			if(!v) v = ++tot; s[v] = s2;
			Hash[s2] = v;
			adde(u, v); adde(v, u);
		}
		tarjan(1, 0);
		int fff = 1;
		for(int i = 1; i <= tot; i++) if(!dfn[i]) {fff = 0;break;}
		if(!fff) {
			printf("0
"); continue;
		}
		for(int i = 1; i <= nume; i++) if(f[i]) cnt++;
		cnt /= 2;
		cout << cnt << endl;
		for(int i = 1; i < nume; i += 2) {
			if(f[i]) cout << s[e[((i - 1) ^ 1) + 1].to] << " " << s[e[i].to] << endl;
		}
	}
}