bzoj 1733: [Usaco2005 feb]Secret Milking Machine 神秘的挤奶机 二分+最大流

#include<iostream>
#include<cstdio>
#include<vector>
#include<queue>
#include<cstring>

#define ll long long
#define inf 10000001

using namespace std;

struct node{
int to,num;
}edge[80080];

int fro[80080],to[80080],len[80080];
int n,m,T;
int pn=1;
vector<int>v[220];
int edgenum=0;
int d[220];

void link(int x,int y,int z){
node t;
t.to=y;
t.num=z;
edge[edgenum]=t;
v[x].push_back(edgenum);
edgenum++;
t.to=x;
t.num=z;
edge[edgenum]=t;
v[y].push_back(edgenum);
edgenum++;
}

bool bfs(){
memset(d,0,sizeof(d));
d[1]=1;
queue<int>q;
q.push(1);
while(!q.empty()){
int t=q.front();
q.pop();
if(t==pn)continue;
for(int i=0; i<v[t].size(); i++){
int x=v[t][i];
int to=edge[x].to;
int num=edge[x].num;
if(num==0 || d[to]!=0)continue;
d[to]=d[t]+1;
q.push(to);
}
}
if(d[pn]!=0)return true;
return false;
}

ll dfs(int x,int y){
if(x==pn || y==0)return y;
ll sum=0;
for(int i=0; i<v[x].size(); i++){
int j=v[x][i];
int to=edge[j].to;
int num=edge[j].num;
if(num){
if(d[to]!=d[x]+1)continue;
ll k=dfs(to,min(num,y));
y-=k;
sum+=k;
edge[j].num-=k;
edge[j^1].num+=k;
if(y==0)break;
}
}
if(sum==0)d[x]=0;
return sum;
}

bool check(int x){
long long ans=0;
edgenum=0;
for(int i=1; i<=n; i++)v[i].clear();
for(int i=1; i<=m; i++){
if(len[i]<=x){
link(fro[i],to[i],1);
}
}
while(bfs())ans+=dfs(1,inf);
return ans>=T;
}

int main(){
scanf("%d%d%d",&n,&m,&T);
for(int i=1; i<=m; i++){
scanf("%d%d%d",&fro[i],&to[i],&len[i]);
}
pn=n;
int l=1,r=1000000;
int ans;
while(l<=r){
int mid=(l+r)/2;
if(check(mid))ans=mid,r=mid-1;
else l=mid+1;
}
printf("%d\n",ans);

return 0;
}