[BZOJ2502]清理雪道解题报告|带下界的最小流

program xjt7;
const maxn = 110;maxm = 100010;INF = 10000000007;
var n,m,e,s,t,x,y:int64;
    i,j:longint;
    fa,next,link,w,cost,rec,son:array[-1..maxm]of int64;
    dis,opt,pos,pre,b,lea:array[-1..maxn]of int64;
    vis:array[-1..maxn]of boolean;

function min(a,b:int64):int64;
begin
    if a<b then exit(a) else exit(b);
end;

procedure add(x,y,z,cst:int64);
begin
    inc(e);fa[e]:=y;next[e]:=link[x];link[x]:=e;w[e]:=z;cost[e]:=cst;rec[e]:=e+1;son[e]:=x;
    inc(e);fa[e]:=x;next[e]:=link[y];link[y]:=e;w[e]:=0;cost[e]:=-cst;rec[e]:=e-1;son[e]:=y;
end;

function spfa:boolean;
var head,tail,x,j:int64;
begin
    fillchar(vis,sizeof(vis),true);
    fillchar(dis,sizeof(dis),63);
    head:=0;tail:=1;opt[1]:=s;dis[s]:=0;vis[s]:=false;
    while head<>tail do
    begin
        head:=(head+1) mod maxn;
        x:=opt[head];j:=link[x];
        while j<>0 do
        begin
            if (w[j]>0)and(dis[x]+cost[j]<dis[fa[j]]) then
            begin
                dis[fa[j]]:=dis[x]+cost[j];pre[fa[j]]:=j;
                if vis[fa[j]] then
                begin
                    vis[fa[j]]:=false;
                    tail:=(tail+1) mod maxn;
                    opt[tail]:=fa[j];
                end;
            end;
            j:=next[j];
        end;
        vis[x]:=true;
    end;
    if dis[t]<>dis[t+1] then exit(true);
    exit(false);
end;

procedure MCMF;
var sum,u,mn,ans:int64;
begin
    ans:=0;
    while spfa do
    begin
        sum:=0;
        u:=t;mn:=INF;
        while u<>s do
        begin
            mn:=min(mn,w[pre[u]]);
            u:=son[pre[u]];
        end;
        u:=t;
        while u<>s do
        begin
            inc(sum,mn*cost[pre[u]]);
            dec(w[pre[u]],mn);inc(w[rec[pre[u]]],mn);
            u:=son[pre[u]];
        end;
        if sum>0 then break;
        inc(ans,mn);
    end;
    writeln(ans);
end;

begin
    //assign(input,‘xjt7.in‘);reset(input);
    readln(n);
        fillchar(b,sizeof(b),0);
    for i:=1 to n do
    begin
        read(lea[i]);
        for j:=1 to lea[i] do
        begin
            read(y);add(i,y,1,-INF);add(i,y,INF,0);
            inc(b[y]);
        end;
        readln;
    end;
    s:=0;t:=n+1;
    for i:=1 to n do if b[i]=0 then add(s,i,INF,1);
    for i:=1 to n do if lea[i]=0 then add(i,t,INF,1);
    MCMF;
end.