Tarjian求無向圖橋模闆 UVA-796 Critical Links

​​UVA-796 Critical Links​​

題目

給一個無向圖，求所有橋，按第一個頂點升序輸出。

分析

代碼

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
typedef long long ll;
const int mod = 1e4 + 1;
const int N = 1e4 + 10;

int n;
int dfs_clock, pre[N], low[N];      // 時間戳、開始點、結束點
int to[N], ne[N], h[N], idx;        // 存圖

int bri[N];         

void add(int a, int b){
    to[idx] = b, ne[idx] = h[a], h[a] = idx++;
}

void dfs(int u, int fa){
    pre[u] = low[u] = ++dfs_clock;
    for (int i = h[u]; ~i; i = ne[i])
    {
        int v = to[i];
        // if(v == fa)
        //     continue;
        if(!pre[v]){
            dfs(v, u);
            low[u] = min(low[u], low[v]);
            if(low[v] > pre[u]){
                bri[i] = 1;         // u -> v
                bri[i ^ 1] = 1;     // v -> u
            }
        }else if(v != fa && low[u] > pre[v])
            low[u] = pre[v];
    }
}

void find_bri(){
    dfs_clock = 0;
    memset(pre, 0, sizeof(pre));
    memset(low, 0, sizeof(low));
    memset(bri, 0, sizeof(bri));
    for(int i = 0; i < n; i++){
        if(!pre[i])
            dfs(i, -1);
    }
    vector<pair<int, int>> ans;

    for(int u = 0; u < n; u++){
        for (int i = h[u]; ~i; i = ne[i]){
            if(bri[i] && to[i] > u)
                ans.push_back(make_pair(u, to[i]));
        }
    }
    
    printf("%d critical links\n", ans.size());
    sort(ans.begin(), ans.end());
    for(int i = 0; i < ans.size(); i++){
        printf("%d - %d\n", ans[i].first, ans[i].second);
    }
    puts("");
}

int main()
{
    while(~scanf("%d", &n)){
        memset(h, -1, sizeof(h));
        idx = 0;
        for(int i = 0, u, v, k; i < n; i++){
            scanf("%d (%d)", &u, &k);
            for(int i = 0; i < k; i++){
                scanf("%d", &v);
                if(v <= u)
                    continue;
                add(u, v), add(v, u);
            }
        }
        find_bri();
    }
    return 0;
}