Luogu P2057 [SHOI2007]善意的投票
题目链接 \(Click\) \(Here\)
考虑模型转换。变成文理分科二选一带收益模型,就一波带走了。
如果没有见过这个模型的话,这里讲的很详细。
#include <bits/stdc++.h>
using namespace std;
#define LL long long
const int N = 400010;
const int M = 800010;
const int INF = 0x3f3f3f3f;
const LL INFF = 0x3f3f3f3f3f3f3f3f;
int n, m, cnt = -1, head[N];
struct edge {
int nxt, to; LL f;
}e[M];
void add_len (int u, int v, LL f) {
e[++cnt] = (edge) {head[u], v, f}; head[u] = cnt;
e[++cnt] = (edge) {head[v], u, 0}; head[v] = cnt;
}
int node (int x) {return x;}
int f1 (int x) {return n + m * 0 + x;}
int f2 (int x) {return n + m * 1 + x;}
queue <int> q;
int cur[N], deep[N];
bool bfs (int s, int t) {
memcpy (cur, head, sizeof (head));
memset (deep, 0x3f, sizeof (deep));
deep[s] = 0; q.push (s);
while (!q.empty ()) {
int u = q.front (); q.pop ();
for (int i = head[u]; ~i; i = e[i].nxt) {
int v = e[i].to;
if (deep[v] == INF && e[i].f) {
deep[v] = deep[u] + 1;
q.push (v);
}
}
}
return deep[t] != INF;
}
LL dfs (int u, int t, LL lim) {
if (u == t || !lim) {
return lim;
}
int tmp = 0, flow = 0;
for (int &i = cur[u]; ~i; i = e[i].nxt) {
int v = e[i].to;
if (deep[v] == deep[u] + 1) {
tmp = dfs (v, t, min (lim, e[i].f));
lim -= tmp;
flow += tmp;
e[i ^ 0].f -= tmp;
e[i ^ 1].f += tmp;
if (!lim) break;
}
}
return flow;
}
int main () {
memset (head, -1, sizeof (head));
cin >> n >> m;
int s = f2 (m + 1), t = f2 (m + 2);
for (int i = 1; i <= n; ++i) {
static int cho;
cin >> cho;
if (cho == 0) {
add_len (s, node (i), INF + 1);
add_len (node (i), t, INF + 0);
} else {
add_len (s, node (i), INF + 0);
add_len (node (i), t, INF + 1);
}
//s -> 0, t -> 1
}
for (int i = 1; i <= m; ++i) {
static int x, y;
cin >> x >> y;
add_len (s, f1 (i), 1); add_len (f1 (i), node (x), INFF); add_len (f1 (i), node (y), INFF);
add_len (f2 (i), t, 1); add_len (node (x), f2 (i), INFF); add_len (node (y), f2 (i), INFF);
}
LL min_cut = 0;
while (bfs (s, t)) {
min_cut += dfs (s, t, INFF);
}
LL ans = (min_cut - n * INF - m);
cout << ans << endl;
}