1797: [Ahoi2009]Mincut 最小割

链接

分析：

　　

代码：

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<iostream>
#include<cmath>
#include<cctype>
#include<queue>
#include<vector>
#include<set>
#include<map>
using namespace std;
typedef long long LL;
 
inline int read() {
    int x=0,f=1;char ch=getchar();for(;!isdigit(ch);ch=getchar())if(ch=='-')f=-1;
    for(;isdigit(ch);ch=getchar())x=x*10+ch-'0';return x*f;
}
 
const int N = 4005, INF = 1e9;
struct Edge { int fr, to, nxt, cap; } e[200005]; 
int head[N], cur[N], dis[N], q[N], low[N], dfn[N], sk[N], bel[N];
int Index, top, S, T, En = 1, n, m, cnt;
bool vis[N];
 
inline void add_edge(int u,int v,int w) {
    ++En; e[En].fr = u, e[En].to = v, e[En].cap = w, e[En].nxt = head[u]; head[u] = En;
    ++En; e[En].fr = v, e[En].to = u, e[En].cap = 0, e[En].nxt = head[v]; head[v] = En;
}
bool bfs() {
    for (int i = 1; i <= n; ++i) cur[i] = head[i], dis[i] = -1;
    int L = 1, R = 0;
    q[++R] = S; dis[S] = 0;
    while (L <= R) {
        int u = q[L ++];
        for (int i = head[u]; i; i = e[i].nxt) {
            int v = e[i].to;
            if (dis[v] == -1 && e[i].cap > 0) {
                dis[v] = dis[u] + 1; q[++R] = v;
                if (v == T) return true;
            }
        }
    }
    return false;
}
int dfs(int u,int flow) {
    if (u == T) return flow;
    int used = 0, tmp;
    for (int &i = cur[u]; i; i = e[i].nxt) {
        int v = e[i].to;
        if (dis[v] == dis[u] + 1 && e[i].cap > 0) {
            tmp = dfs(v, min(flow - used, e[i].cap));
            if (tmp > 0) {
                used += tmp; e[i].cap -= tmp; e[i ^ 1].cap += tmp;
                if (used == flow) break;
            }
        }
    }
    if (used != flow) dis[u] = -1;
    return used;
}
int dinic() {
    int ans = 0;
    while (bfs()) ans += dfs(S, INF);
    return ans;
}
void tarjan(int u) {
    low[u] = dfn[u] = ++Index; 
    sk[++top] = u, vis[u] = 1;
    for (int i = head[u]; i; i = e[i].nxt) {
        int v = e[i].to;
        if (e[i].cap) {
            if (!dfn[v]) {
                tarjan(v);
                low[u] = min(low[u], low[v]);
            }
            else if (vis[v]) low[u] = min(low[u], dfn[v]);
        }
    }
    if (low[u] == dfn[u]) {
        ++cnt;
        do {
            bel[sk[top]] = cnt;
            vis[sk[top]] = false;
            top --;
        } while (sk[top + 1] != u);
    }
}
int main() {
    n = read(), m = read(); S = read(), T = read();
    for (int i = 1; i <= m; ++i) {
        int u = read(), v = read(), w = read();
        add_edge(u, v, w);
    }
    dinic();
    for (int i = 1; i <= n; ++i) 
    if (!dfn[i]) tarjan(i);
    for (int i = 2; i <= En; i += 2) {
        if (e[i].cap > 0) puts("0 0");
        else {
            printf(bel[e[i].fr] != bel[e[i].to] ? "1 " : "0 ");
            puts(bel[e[i].fr] == bel[S] && bel[e[i].to] == bel[T] ? "1" : "0");
        }
    }
    return 0;
}