USACO 4.2.1 网络流 最高标号预流推进(HLPP)

/*
ID: shenxyy1
TASK: ditch
LANG: C++11
*/
#include<bits/stdc++.h>

using namespace std;

#define set(x) Set(x)
#define REP(i,j,k) for (int i=(j),_end_=(k);i<=_end_;++i)
#define DREP(i,j,k) for (int i=(j),_start_=(k);i>=_start_;--i)
#define debug(...) fprintf(stderr,__VA_ARGS__)
#define mp make_pair
#define x first
#define y second
#define pb push_back
#define SZ(x) (int((x).size()-1))
#define ALL(x) ((x).begin()+1),(x).end()

template<typename T> inline bool chkmin(T &a,const T &b){ return a > b ? a = b, 1 : 0; }
template<typename T> inline bool chkmax(T &a,const T &b){ return a < b ? a = b, 1 : 0; }

typedef long long LL;
typedef pair<int,int> node;

const int dmax=1010,oo=0x3f3f3f3f;

int n,m;

int a[dmax][dmax] , ans;

int d[dmax],e[dmax];

priority_queue <node> q;

inline bool operator >(node a,node b){ return a.y>b.y; }

bool p[dmax];

void Set(int x){ p[x]=1; }
void unset(int x){ p[x]=0; }
bool check(int x){ return x!=1 && x!=n && !p[x] && e[x]>0; }

void preflow(){
e[1]=oo;
d[1]=n-1;
q.push(mp(1,n-1));
set(1);
while (!q.empty()){
bool flag=1;
int k=q.top().x;
q.pop(),unset(k);
DREP(i,n,1)
if ((d[k]==d[i]+1 || k==1) && a[k][i]>0){
flag=0;
int t=min(a[k][i],e[k]);
e[k]-=t;
a[k][i]-=t;
e[i]+=t;
a[i][k]+=t;
if (check(i)){
q.push(mp(i,d[i]));
set(i);
}
if (e[k]==0) break;
}
if (flag){
d[k]=oo;
REP(i,1,n)
if (a[k][i]>0)
chkmin(d[k],d[i]+1);
}
if (check(k)){
q.push(mp(k,d[k]));
set(k);
}
}
ans=e
;
}

int main(){
freopen("ditch.in","r",stdin);
freopen("ditch.out","w",stdout);
scanf("%d%d",&m,&n);
REP(i,1,m){
int x,y,z;
scanf("%d%d%d",&x,&y,&z);
a[x][y]+=z;
}
preflow();
printf("%d\n",ans);
return 0;
}