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\n"); 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;
        }
    }
}
posted @ 2018-04-02 15:01  Mr_Wolfram  阅读(...)  评论(...编辑  收藏