EK算法 & Dinic

2. 基本算法

EK : (O(nm^2))

Dinic: (O(n^2m))

但其实上界非常宽松，一般 EK 能处理 (10^3-10^4) 的数据，Dinic (10^4-10^5)

EK算法

一般求最大流用不到，求最小费用流的时候 EK是核心算法

dinic

最大流

(EK)

/*
 * @Author: zhl
 * @Date: 2020-10-20 10:25:08
 */
#include<bits/stdc++.h>
#define rep(i,a,b) for(int i = a;i <= b;i++)
#define repE(i,u) for(int i = head[u];~i;i = E[i].next)
using namespace std;
const int N = 1e3 + 10;
const int M = 2e4 + 10;
const int inf = 0x3f3f3f3f;
struct Edge {
	int to, flow, next;
}E[M];

int head[N], tot;
void addEdge(int from, int to, int flow) {
	E[tot] = Edge{ to,flow,head[from] };
	head[from] = tot++;
}

int n, m, S, T;
int d[N], pre[N];
bool vis[N];

bool bfs() {
	queue<int>Q;
	memset(vis, 0, sizeof vis);
	Q.push(S); vis[S] = 1;
	d[S] = inf;
	while (!Q.empty()) {
		int tp = Q.front(); Q.pop();
		repE(i, tp) {
			int u = E[i].to;
			if (!vis[u] and E[i].flow) {
				vis[u] = 1;
				d[u] = min(d[tp], E[i].flow);
				pre[u] = i;
				if (u == T)return true;
				Q.push(u);
			}
		}
	}
	return false;
}

int EK() {
	int ans = 0;
	while (bfs()) {
		ans += d[T];
		for (int i = T; i != S; i = E[pre[i] ^ 1].to) {
			E[pre[i]].flow -= d[T];
			E[pre[i] ^ 1].flow += d[T];
		}
	}
	return ans;
}
int main() {
	scanf("%d%d%d%d", &n, &m, &S, &T);
	memset(head, -1, sizeof head);

	rep(i, 1, m) {
		int a, b, c;
		scanf("%d%d%d", &a, &b, &c);
		addEdge(a, b, c);
		addEdge(b, a, 0);
	}

	printf("%d
", EK());
}

(Dinic)

/*
 * @Author: zhl
 * @Date: 2020-10-20 11:09:59
 */
#include<bits/stdc++.h>
//#define int long long
using namespace std;

const int N = 1e4 + 10, M = 1e5 + 10, inf = 1e9;
int n, m, s, t, tot, head[N];
int ans, dis[N], cur[N];

struct Edge {
	int to, next, flow;
}E[M<<1];

void addEdge(int from, int to, int w) {
	E[tot] = Edge{ to,head[from],w };
	head[from] = tot++;
	E[tot] = Edge{ from,head[to],0 };
	head[to] = tot++;
}

int bfs() {
	for (int i = 1; i <= n; i++) dis[i] = -1;
	queue<int>Q;
	Q.push(s);
	dis[s] = 0;
	cur[s] = head[s];

	while (!Q.empty()) {
		int u = Q.front();
		Q.pop();
		for (int i = head[u]; ~i; i = E[i].next) {
			int v = E[i].to;
			if (E[i].flow && dis[v] == -1) {
				Q.push(v);
				dis[v] = dis[u] + 1;
				cur[v] = head[v];
				if (v == t)return 1; //分层成功
			}
		}
	}
	return 0;
}

int dfs(int x, int sum) {
	if (x == t)return sum;
	int k, res = 0;
	for (int i = cur[x]; ~i && res < sum; i = E[i].next) {
		cur[x] = i;
		int v = E[i].to;
		if (E[i].flow > 0 && (dis[v] == dis[x] + 1)) {
			k = dfs(v, min(sum, E[i].flow));
			if (k == 0) dis[v] = -1; //不可用
			E[i].flow -= k;E[i ^ 1].flow += k;
			res += k;sum -= k;
		}
	}
	return res;
}

int Dinic() {
	int ans = 0;
	while (bfs()) {
        ans += dfs(s,inf);
	}
	return ans;
}

signed main() {
	scanf("%d%d%d%d",&n,&m,&s,&t);
	memset(head, -1, sizeof(int) * (n + 10));
	for (int i = 1; i <= m; i++) {
		int a, b, w;
		scanf("%d%d%d",&a, &b, &w);
		addEdge(a, b, w);
	}
	printf("%d
",Dinic());
}