【luogu P2936 [USACO09JAN]全流Total Flow】 题解

题目链接：https://www.luogu.org/problemnew/show/P2936
菜

#include <queue>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
const int maxn = 10000;
const int inf = 1e9;
int n, m, s, t, maxflow, deep[maxn];
struct edge{
	int next, to, len;
}e[maxn<<2];
int head[maxn], cnt = -1, cur[maxn];
queue<int> q;
void add(int u, int v, int w, int flag)
{
	e[++cnt].next = head[u];
	e[cnt].to = v;
	if(flag) e[cnt].len = w;
	head[u] = cnt;
}
bool bfs(int s, int t)
{
	memset(deep, 0x7f, sizeof(deep));
	while(!q.empty()) q.pop();
	for(int i = 1; i <= n; i++) cur[i] = head[i];
	q.push(s); deep[s] = 0;
	while(!q.empty())
	{
		int now = q.front(); q.pop();
		for(int i = head[now]; i != -1; i = e[i].next)
		{
			if(deep[e[i].to] > inf && e[i].len)
			{
				deep[e[i].to] = deep[now] + 1;
				q.push(e[i].to);
			}
		}
	}
	if(deep[t] < inf) return true;
	else return false;
}
int dfs(int now, int t, int limit)
{
	if(!limit || now == t) return limit;
	int flow = 0, f;
	for(int i = head[now]; i != -1; i = e[i].next)
	{
		if(deep[e[i].to] == deep[now] + 1 && (f = dfs(e[i].to, t, min(e[i].len, limit))))
		{
			flow += f;
			limit -= f;
			e[i].len -= f;
			e[i^1].len += f;
			if(!limit) break;
		}
	}
	return flow;
}
void Dinic(int s, int t)
{
	while(bfs(s, t))
	maxflow += dfs(s, t, inf);
}
int main()
{
	memset(head, -1, sizeof(head));
	scanf("%d",&m);
	s = 1, t = 26;
	for(int i = 1; i <= m; i++)
	{
		char a, b; int u, v, w;
		cin>>a>>b>>w;
		u = a-'A'+1;
		v = b-'A'+1;
		//cout<<u<<" "<<v<<endl;
		add(u, v, w, 1);
		add(v, u, w, 0);
	}
	Dinic(s, t);
	printf("%d\n",maxflow);
	return 0;
}