BZOJ 1614: [Usaco2007 Jan]Telephone Lines架设电话线 二分答案 最短路

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
#include <queue>
#define N 1000+50
#define M 20000+50
using namespace std;
int head
,dis
;
const int inf = 0x7fffff;
int pre
,from
;
struct graph
{
int next,to,val;
int deval;
graph () {}
graph (int _next,int _to,int _val)
:next(_next),to(_to),val(_val){}
}edge[M];
int cnt = 0;
inline void add(int x,int y,int z)
{
edge[++cnt] = graph(head[x],y,z);
head[x]=cnt;
edge[++cnt] = graph(head[y],x,z);
head[y]=cnt;
}
int S,T;
bool in
;
queue<int>Q;
void spfa()
{
memset(dis,0x3f,sizeof dis);
memset(in,false,sizeof in);
memset(pre,0,sizeof pre);
memset(from,0,sizeof from);
Q.push(S),dis[S]=0;in[S]=1;
while(!Q.empty())
{
int tt=Q.front();Q.pop();in[tt]=false;
for(int i=head[tt];i;i=edge[i].next)
if(dis[edge[i].to]>dis[tt]+edge[i].deval)
{
dis[edge[i].to]=dis[tt]+edge[i].deval;
pre[edge[i].to] = tt;from[edge[i].to] = i;
if(!in[edge[i].to])
{
Q.push(edge[i].to);
in[edge[i].to]=1;
}
}
}
}
int n,m,k;
bool check(int mid)
{
for(int i=1;i<=cnt;++i)
{
if(edge[i].val<=mid)edge[i].deval = 0;
else edge[i].deval = 1;
}
spfa();
if(dis[T]>k)
return false;
return true;
}
inline int read()
{
int x=0,f=1;char ch = getchar ();
while (ch < '0' || ch > '9') {if (ch == '-')f=-1;ch = getchar();}
while (ch >='0' && ch <='9') {x=(x<<1)+(x<<3)+ch -'0';ch = getchar();}
return x*f;
}
int main()
{
n=read(),m=read(),k=read();
S = 1,T = n;
int l = 0 , r = 0;
for(int i=1;i<=m;++i)
{
int x=read(),y=read(),z=read();
add(x,y,z); l = min(l,z),r = max(r,z);
}
int ans = inf;
while(l<r)
{
int mid = (l+r)>>1;
if(check(mid))
{
int now = n;
int tmp = 0;
while(now ^ 1)
{
if(edge[from[now]].deval == 0)
tmp = max(tmp,edge[from[now]].val);
now = pre[now];
}
ans = min(ans,tmp);
r = mid;
}
else
l = mid + 1;
}
printf("%d\n",ans == inf ? -1 : ans);
}