POJ 2421[Constructing Roads]

不解释的MST，把已经有道路的两个村庄之间建一条长度为0的边，然后Kruskal。我开始是把已经有路的村庄合并，WA了

p.s.这个题的Discuss里面楼要倒了……

Var
        a,b,e,f:array[1..100000]of longint;
        n,m,sum,ans:longint;
Procedure addgraph(x,y,z:longint);
 begin
        inc(m);
        a[m]:=x;b[m]:=y;e[m]:=z;
 end;
Procedure init;
 var
        i,j,pre,k:longint;
 begin
        readln(n);
        fillchar(f,sizeof(f),0);
        for i:=1 to n do
                begin
                for j:=1 to n do
                        begin
                                read(pre);
                                addgraph(i,j,pre);
                        end;
                readln;
                end;
        readln(k);
        for i:=1 to k do
                begin
                        read(j,pre);
                        addgraph(j,pre,0);
                end;
 end;
Procedure qsort(l,r:longint);
 var
 i,j,x,y:longint;
 begin
        i:=l;j:=r;x:=e[(l+r)shr 1];
        Repeat
                while e[i]<x do inc(i);
                while e[j]>x do dec(j);
                if i<=j then
                        begin
                                y:=a[i];a[i]:=a[j];a[j]:=y;
                                y:=b[i];b[i]:=b[j];b[j]:=y;
                                y:=e[i];e[i]:=e[j];e[j]:=y;
                                inc(i);dec(j);
                        end;
        Until i>j;
        if i<r then qsort(i,r);
        if l<j then qsort(l,j);
 end;
Function find(x:longint):longint;
 var
        k:longint;
 begin
        k:=x;
        while f[k]<>0 do k:=f[k];
        find:=k;
 end;
Procedure kruskal;
 var
        i,j,p,q:longint;
 begin
        qsort(1,m);
        for i:=1 to m do
                begin
                        p:=find(a[i]);
                        q:=find(b[i]);
                        if p<>q then
                                begin
                                        f[q]:=p;
                                        inc(sum,e[i]);
                                        inc(ans);
                                end;
                if ans>=n-1 then break;
                end;
        writeln(sum);
 end;
Begin


        init;
        kruskal;

End.