[UOJ]210 寻找罪犯 2-Sat 前缀和优化

#include<bits/stdc++.h>
#define acce register int
#define boc register char
using namespace std;
const int maxn = 5e5 + 5;
int all, n, m, num, cnt, top, idx, ret;
int h[maxn], scc[maxn], low[maxn], s[maxn], p[maxn], dfn[maxn], pre[maxn];
inline const int read() {
acce x = 0;
boc ch = getchar();
while (ch < '0' || ch > '9') ch = getchar();
while (ch >= '0' && ch <= '9') x = (x << 3) + (x << 1) + ch - '0', ch = getchar();
return x;
}
struct edge{ int nxt, v; }e[maxn * 2];
inline void add(int u, int v) {
e[++ num].v = v, e[num].nxt = h[u], h[u] = num;
e[++ num].v = u ^ 1, e[num].nxt = h[v ^ 1], h[v ^ 1] = num;
}
void dfs(int u) {
s[++ top] = u;
low[u] = dfn[u] = ++ idx;
for (acce i = h[u]; i; i = e[i].nxt) {
int v = e[i].v;
if (!dfn[v])
dfs(v), low[u] = min(low[u], low[v]);
else if (!scc[v]) low[u] = min(low[u], dfn[v]);
}
if (dfn[u] == low[u]) {
++ cnt;
while (s[top + 1] != u)
scc[s[top --]] = cnt;
}
}
inline const int id(const int &x, const int &d) { return x << 1 | d;}
inline const int say(const int &x, const int &d) { return (n + x) << 1 | d;}
int main() {
acce i, x, y, t;
n = read(), m = read();
all = say(m + 1, 1) + 1;
for (i = 1; i <= n; ++ i) pre[i] = 1;
for (i = 2; i <= m + 1; ++ i) {
x = read(), y = read(), t = read(), t = !t;
add(id(y, !t), say(pre[x], 1));
add(say(i, 1), id(y, t));
add(say(i, 1), say(pre[x], 1));
pre[x] = i;
}
for (i = 1; i <= n; ++ i) add(say(pre[i], 0), id(i, 1));
for (i = all; i; -- i)
if (!dfn[i]) dfs(i);
for (i = 1; i <= all; i += 2)
if (scc[i] == scc[i ^ 1]) return puts("Impossible"), 0;
for (i = 1; i <= n; ++ i)
if (scc[id(i, 1)] < scc[id(i, 0)]) p[++ ret] = i;
printf("%d\n", ret);
for (i = 1; i <= ret; ++ i)
printf("%d ", p[i]);
return 0;
}