架设电话线 二分+图论

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<algorithm>

#define ll long long
#define il inline
#define db double

using namespace std;

il int gi()
{
	int x=0,y=1;
	char ch=getchar();
	while(ch<‘0‘||ch>‘9‘)
		{
			if(ch==‘-‘)
				y=-1;
			ch=getchar();
		}
	while(ch>=‘0‘&&ch<=‘9‘)
		{
			x=x*10+ch-‘0‘;
			ch=getchar();
		}
	return x*y;
}

int n,m,k;

int head[100045],cnt=-1;

struct edge
{
	int next,to,lon,cap;
}e[100045];

il void add(int from,int to,int lon)
{
	e[++cnt].next=head[from];
	e[cnt].to=to;
	e[cnt].lon=lon;
	head[from]=cnt;
}

int dist[10045];

bool vis[10045];

int t[10045];

int headd,tail;

il bool check(int mid)
{
	for(int i=0;i<=2*m;i++)
		if(e[i].lon>mid)
			e[i].cap=1;
		else
			e[i].cap=0;
	
	memset(dist,127/3,sizeof(dist));
	memset(vis,0,sizeof(vis));

	headd=0,tail=1;

	t[0]=1;
	dist[1]=0;
	
	while(headd!=tail)
		{
			int h=t[headd++];
			vis[h]=0;
			int r=head[h];
			while(r!=-1)
				{
					int now=e[r].to;
					if(dist[now]>dist[h]+e[r].cap)
						{
							dist[now]=dist[h]+e[r].cap;
							if(!vis[now])
								{
									vis[now]=1;
									t[tail++]=now;
								}

						}
					r=e[r].next;
				}

		}

	if(dist[n]<=k&&dist[n]!=707406378)
		return 1;
	else
		return 0;
}

int main()
{
	freopen("phoneline.in","r",stdin);
	freopen("phoneline.out","w",stdout);

	memset(head,-1,sizeof(head));

	n=gi(),m=gi(),k=gi();

	int x,y,z;
	for(int i=1;i<=m;i++)
		{
			x=gi(),y=gi(),z=gi();
			add(x,y,z);
			add(y,x,z);
		}

	int l=0,r=1000000;
	int ans=-1;
	while(l<=r)
		{
			int mid=(l+r)>>1;
			if(check(mid))
				{
					ans=mid;
					r=mid-1;
				}
			else
				l=mid+1;
		}

	printf("%d\n",ans);

	return 0;
}