2017"百度之星"程序设计大赛

全局最小割 Stoer-Wagner （SW算法）优化

优化吃藕了，感谢放宽时限，感谢平板电视 (pb_ds)

#include <iostream>
#include <cstdio>
#include <cstring>
#include <ext/pb_ds/priority_queue.hpp>

typedef long long ll;
typedef __gnu_pbds::priority_queue<std::pair<int, int>, std::less<std::pair<int, int> >, __gnu_pbds::binomial_heap_tag >pq;

struct Edge {int from, to, nxt, val;};

const int maxn = 3e3 + 7;
const int maxm = 4e5 + 7;
const int inf = 1000000000;
int n, r;
bool vis[maxn], bin[maxn];
Edge e[maxm * 10 + 7];
int head[maxn], id[maxn][maxn], esz;

inline ll read() {
  int x = 0, f = 1; char ch = getchar();
  while(ch < '0' || ch > '9') {if(ch == '-')f = -1; ch = getchar();}
  while(ch >= '0' && ch <= '9') {x = x * 10 + ch - '0'; ch = getchar();}
  return x * f;
}

inline void add_edge(int from, int to, int val) {
  if(id[from][to] == -1) {
    e[esz].from = from, e[esz].to = to, e[esz].val = val, e[esz].nxt = head[from];
    id[from][to] = esz; head[from] = esz++;
  } else {
    e[id[from][to]].val += val;
  }
}

inline void init() {
  memset(bin, false, sizeof(bin));
  memset(head, -1, sizeof(head));
  memset(id, -1, sizeof(id));
  esz = 0;
}


pq que;
pq::point_iterator it[maxn];
inline int contract(int &s, int &t) {          // Find s,t
  memset(vis, false, sizeof(vis));
  int i, j, k, mincut, maxc;
  while(!que.empty()) que.pop();
  for(i = 1; i <= n; ++i)
    if(!bin[i])it[i] = que.push(std::make_pair(0, i));
    else it[i] = nullptr;
  for(i = 1; i <= n; i++) {
    k = -1; maxc = -1;
    if(que.empty()) return mincut;
    k = que.top().second, maxc = que.top().first; que.pop();
    s = t;  t = k; mincut = maxc; vis[k] = true;
    for(j = head[k]; ~j; j = e[j].nxt) {
      int v = e[j].to, w = e[j].val;
      if(!bin[v] && !vis[v]) {
        que.modify(it[v], std::make_pair((it[v]->first) + w, v));
      }
    }
  }
  return mincut;
}

inline int Stoer_Wagner() {
  int mincut, i, j, s, t, ans;
  for(mincut = inf, i = 1; i < n; i++) {
    ans = contract(s, t);
    bin[t] = true;
    if(mincut > ans) mincut = ans;
    if(mincut == 0)return 0;
    for(j = 1; j <= n; j++) if(!bin[j]) {
        if(id[j][t] == -1)continue;
        if(id[s][j] == -1)add_edge(s, j, e[id[j][t]].val);
        else e[id[s][j]].val += e[id[j][t]].val;
        if(id[j][s] == -1)add_edge(j, s, e[id[j][t]].val);
        else e[id[j][s]].val += e[id[j][t]].val;
      }
  }
  return mincut;
}

int main() {
  int m;
  while(scanf("%d%d", &n, &m) != -1) {
    init();
    for(int i = 1; i <= m; i++) {
      int a, b, c;
      a = read(), b = read(), c = read();
      add_edge(a, b, c); add_edge(b, a, c);
    }
    printf("%d
", Stoer_Wagner());
  }
  return 0;
}