紫书 习题 11-3 UVa 820 （最大流裸题）

注意这道题是双向边， 然后直接套模板就ok了。

#include<cstdio>
#include<algorithm>
#include<vector>
#include<queue>
#include<cstring>
#define REP(i, a, b) for(int i = (a); i < (b); i++)
using namespace std;

const int MAXN = 112;
struct Edge
{
	int from, to, cap, flow;
	Edge(int from = 0,int to = 0,int cap = 0,int flow = 0):from(from),to(to),cap(cap),flow(flow){}
};
vector<Edge> edges;
vector<int> g[MAXN];
int h[MAXN], cur[MAXN], s, t, n, m;

void AddEdge(int from, int to, int cap)
{
	edges.push_back(Edge(from, to, cap, 0));
	edges.push_back(Edge(to, from, 0, 0));
	g[from].push_back(edges.size() - 2);
	g[to].push_back(edges.size() - 1);
}

bool bfs()
{
	memset(h, 0, sizeof(h));
	queue<int> q;
	q.push(s);
	h[s] = 1;
	
	while(!q.empty())
	{
		int x = q.front(); q.pop();
		REP(i, 0, g[x].size())
		{
			Edge& e = edges[g[x][i]];
			if(e.cap > e.flow && !h[e.to])
			{
				h[e.to] = h[x] + 1;
				q.push(e.to);
			}
		}
	}
	
	return h[t];
}

int dfs(int x, int a)
{
	if(x == t || a == 0) return a;
	int flow = 0, f;
	for(int i = cur[x]; i < g[x].size(); i++)
		{
			Edge& e = edges[g[x][i]];
			if(h[x] + 1 == h[e.to] && (f = dfs(e.to, min(a, e.cap - e.flow))) > 0)
			{
				e.flow += f;
				edges[g[x][i] ^ 1].flow -= f;
				flow += f;
				if((a -= f) == 0) break;
			}
		}
	return flow;
}

int solve()
{
	int ret = 0;
	while(bfs()) memset(cur, 0, sizeof(cur)), ret += dfs(s, 1e9);
	return ret;
}

int main()
{
	int kase = 0;
	while(~scanf("%d", &n) && n)
	{
		REP(i, 1, n + 1) g[i].clear();
		edges.clear();
		
		scanf("%d%d%d", &s, &t, &m);
		while(m--)
		{
			int u, v, f;
			scanf("%d%d%d", &u, &v, &f);
			AddEdge(u, v, f);
			AddEdge(v, u, f);
		}
		printf("Network %d\nThe bandwidth is %d.\n\n", ++kase, solve());

	}
	return 0;
}