[HDOJ4738]Caocao's Bridges（双联通分量，割边，tarjan）

Posted 2020-07-14

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=4738

给一张无向图，每一条边都有权值。找一条割边，使得删掉这条边双连通分量数量增加，求权值最小那条。

注意有重边，ACEveryDay里群巨给的意见是tarjan的时候记录当前点是从哪条边来的。

注意假如桥的权值是0的时候也得有一个人去炸……

 

  1 /*
  2 ━━━━━┒ギリギリ♂ eye！
  3 ┓┏┓┏┓┃キリキリ♂ mind！
  4 ┛┗┛┗┛┃＼○／
  5 ┓┏┓┏┓┃ /
  6 ┛┗┛┗┛┃ノ)
  7 ┓┏┓┏┓┃
  8 ┛┗┛┗┛┃
  9 ┓┏┓┏┓┃
 10 ┛┗┛┗┛┃
 11 ┓┏┓┏┓┃
 12 ┛┗┛┗┛┃
 13 ┓┏┓┏┓┃
 14 ┃┃┃┃┃┃
 15 ┻┻┻┻┻┻
 16 */
 17 #include <algorithm>
 18 #include <iostream>
 19 #include <iomanip>
 20 #include <cstring>
 21 #include <climits>
 22 #include <complex>
 23 #include <fstream>
 24 #include <cassert>
 25 #include <cstdio>
 26 #include <bitset>
 27 #include <vector>
 28 #include <deque>
 29 #include <queue>
 30 #include <stack>
 31 #include <ctime>
 32 #include <set>
 33 #include <map>
 34 #include <cmath>
 35 using namespace std;
 36 #define fr first
 37 #define sc second
 38 #define cl clear
 39 #define BUG puts("here!!!")
 40 #define W(a) while(a--)
 41 #define pb(a) push_back(a)
 42 #define Rint(a) scanf("%d", &a)
 43 #define Rll(a) scanf("%lld", &a)
 44 #define Rs(a) scanf("%s", a)
 45 #define Cin(a) cin >> a
 46 #define FRead() freopen("in", "r", stdin)
 47 #define FWrite() freopen("out", "w", stdout)
 48 #define Rep(i, len) for(int i = 0; i < (len); i++)
 49 #define For(i, a, len) for(int i = (a); i < (len); i++)
 50 #define Cls(a) memset((a), 0, sizeof(a))
 51 #define Clr(a, x) memset((a), (x), sizeof(a))
 52 #define Full(a) memset((a), 0x7f7f, sizeof(a))
 53 #define lp p << 1
 54 #define rp p << 1 | 1
 55 #define pi 3.14159265359
 56 #define RT return
 57 #define lowbit(x) x & (-x)
 58 #define onenum(x) __builtin_popcount(x)
 59 typedef long long LL;
 60 typedef long double LD;
 61 typedef unsigned long long ULL;
 62 typedef pair<int, int> pii;
 63 typedef pair<string, int> psi;
 64 typedef map<string, int> msi;
 65 typedef vector<int> vi;
 66 typedef vector<LL> vl;
 67 typedef vector<vl> vvl;
 68 typedef vector<bool> vb;
 69 
 70 typedef struct Edge {
 71     int u, v, w;
 72     int idx, next;
 73     bool cut;
 74     Edge() {}
 75     Edge(int uu, int vv, int ww, int ii) : u(uu), v(vv), w(ww), idx(ii) {}
 76 }Edge;
 77 const int maxn = 1010;
 78 const int maxm = maxn*maxn;
 79 int n, m;
 80 int bridge;
 81 Edge edge[maxm];
 82 int head[maxn];
 83 int dfn[maxn], low[maxn];
 84 int ecnt;
 85 int ret;
 86 
 87 void adde(int u, int v, int w, int i) {
 88     edge[ecnt] = Edge(u, v, w, i);
 89     edge[ecnt].next = head[u];
 90     edge[ecnt].cut = 0;
 91     head[u] = ecnt++;
 92 }
 93 
 94 void dfs(int u, int d, int p) {
 95     low[u] = dfn[u] = d;
 96     for(int i = head[u]; ~i; i=edge[i].next) {
 97         int v = edge[i].v;
 98         int idx = edge[i].idx;
 99         if(p == idx) continue; 
100         if(!dfn[v]) {
101             dfs(v, d+1, idx);
102             low[u] = min(low[u], low[v]);
103             if(low[v] > dfn[u]) {
104                 bridge++;
105                 edge[i].cut = edge[i^1].cut = 1;
106                 ret = min(ret, edge[i].w);
107             }
108         }
109         else low[u] = min(low[u], dfn[v]);
110     }
111 }
112 
113 int main() {
114     // FRead();
115     int u, v, w;
116     while(~Rint(n) && ~Rint(m) && n + m) {
117         Clr(head, -1); Cls(dfn); Cls(low);
118         ecnt = 0; bridge = 0; ret = 0x7f7f7f;
119         For(i, 1, m+1) {
120             Rint(u); Rint(v); Rint(w);
121             adde(u, v, w, i); adde(v, u, w, i);
122         }
123         int cnt = 0;
124         For(i, 1, n+1) {
125             if(!dfn[i]) {
126                 cnt++;
127                 dfs(i, 0, 0);
128             }
129         }
130         if(ret == 0) ret = 1;
131         if(ret == 0x7f7f7f) ret = -1;
132         if(cnt > 1) ret = 0;
133         printf("%d\n", ret);
134     }
135     RT 0;
136 }