学步园

大意不再赘述。

题目链接：胡老师教你做图论

思路：找这道题，我都在网上找了N久，最后在OPEN OJ里发现了，于是就写啦写，用了我好久的时间哦。。

见另一篇博客：http://blog.csdn.net/wall_f/article/details/8234141

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <string>
using namespace std;

typedef long long LL;

const int MAXN = 10001;
const int MAXM = 1000010;
const int INF = 0x3f3f3f3f;
const LL MAX_ = 99999999999999;

struct Edge
{
	int v, f;
	int next;
}edge[MAXM];

int n, m;
int cnt;
int totP;
int s, t;

int first[MAXN], level[MAXN];
LL d[MAXN][MAXN];
int P[MAXN], C[MAXN];

LL MaxR;

void init()
{
	cnt = 0;
	memset(first, -1, sizeof(first));
}

void Floyd()
{
	for(int k = 1; k <= n; k++)
	for(int i = 1; i <= n; i++) if(d[i][k] != MAX_)
		for(int j = 1; j <= n; j++) d[i][j] = min(d[i][j], d[i][k] + d[k][j]);
}

void read_graph(int u, int v, int f)
{
	edge[cnt].v = v, edge[cnt].f = f;
	edge[cnt].next = first[u], first[u] = cnt++;
	edge[cnt].v = u, edge[cnt].f = 0;
	edge[cnt].next = first[v], first[v] = cnt++;
}

int bfs(int s, int t)
{
	int q[MAXN];
	memset(level, 0, sizeof(level));
	level[s] = 1;
	int front = 0, rear = 1;
	q[front] = s;
	while(front < rear)
	{
		int x = q[front++];
		if(x == t) return 1;
		for(int e = first[x]; e != -1; e = edge[e].next)
		{
			int v = edge[e].v, f = edge[e].f;
			if(!level[v] && f)
			{
				level[v] = level[x] + 1;
				q[rear++] = v;
			}
		}
	}
	return 0;
}

int dfs(int u, int maxf, int t)
{
	if(u == t) return maxf;
	int ret = 0;
	for(int e = first[u]; e != -1; e = edge[e].next)
	{
		int v = edge[e].v, f = edge[e].f;
		if(level[v] == level[u] + 1 && f)
		{
			int Min = min(maxf-ret, f);
			f = dfs(v, Min, t);
			edge[e].f -= f;
			edge[e^1].f += f;
			ret += f;
			if(ret == maxf) return ret;
		}
	}
	return ret;
}

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

void read_case()
{
	totP = 0;
	MaxR = 0;
	s = 0, t = 2*n+1;
	for(int i = 1; i <= n; i++)
	{
		scanf("%d%d", &P[i], &C[i]);
		totP += P[i];
	}
	for(int i = 1; i <= n; i++)
	{
		for(int j = 1; j <= n; j++)
		{
			d[i][j] = (i == j)? 0:MAX_;
		}
	}
	while(m--)
	{
		int u, v;
		LL w;
		scanf("%d%d%lld", &u, &v, &w);
		MaxR += w;
		if(d[u][v] > w)
		{
			d[u][v] = d[v][u] = w;
		}
	}
	Floyd();
}

void build(LL mid)
{
	init();
	for(int i = 1; i <= n; i++) if(P[i])
	{
		read_graph(s, i, P[i]);
	}
	for(int i = 1; i <= n; i++) if(C[i])
	{
		read_graph(i+n, t, C[i]);
	}
	for(int i = 1; i <= n; i++)
	{
		for(int j = 1; j <= n; j++)
		{
			if(d[i][j] <= mid && C[j]) //删除孤立边 
			{
				read_graph(i, j+n, INF);
			}
		}
	}
}

void solve()
{
	read_case();
	int flag = 0;
	LL x = 0, y = MaxR+1;
	while(x <= y)
	{
		LL mid = x+(y-x)/2;
		build(mid);
		int ans = Dinic(s, t);
		if(ans >= totP)
		{
			y = mid-1;
			flag = 1;
		}
		else x = mid+1;
	}
	printf(flag? "%lld\n":"-1\n", x);
}

int main()
{
	while(~scanf("%d%d", &n, &m))
	{
		solve();
	}
	return 0;
}