USACO CONTEST 2002 OPEN 绿组.秘密管道[secret]

 



View Code 
 1 program secret4;
 2 var w,p,ans0:longint;
 3     xx,yy,cost:array[0..20000] of longint;
 4     use:array[0..20000] of boolean;
 5     fa,tf,f:array[0..2000] of longint;
 6 
 7  procedure QuickSort(l,r:longint);
 8  var i,j,k,mid:longint;
 9  begin
10   i:=l; j:=r; mid:=cost[(l+r) div 2];
11   repeat
12    while cost[i]<mid do inc(i);
13    while cost[j]>mid do dec(j);
14    if i<=j then
15    begin
16     k:=cost[i]; cost[i]:=cost[j]; cost[j]:=k;
17     k:=xx[i]; xx[i]:=xx[j]; xx[j]:=k;
18     k:=yy[i]; yy[i]:=yy[j]; yy[j]:=k;
19     inc(i); dec(j);
20    end;
21   until i>j;
22   if l<j then QuickSort(l,j);
23   if i<r then QuickSort(i,r);
24  end;
25 
26  procedure init;
27  var i:longint;
28  begin
29   readln(w,p);
30   for i:=1 to p do read(xx[i],yy[i],cost[i]);
31   QuickSort(1,p);
32  end;
33 
34  function find(x:longint):longint;
35  begin
36   if fa[x]=x then exit(x);
37   fa[x]:=find(fa[x]);
38   exit(fa[x]);
39  end;
40 
41  procedure kruskal;
42  var i,j,fx,fy:longint;
43  begin
44   for i:=1 to w do fa[i]:=i;
45   i:=0; j:=0;
46   while (j<=w-1) and (i<=p) do
47   begin
48    inc(i);
49    if use[i] then
50    begin
51     fx:=find(xx[i]);
52     fy:=find(yy[i]);
53     if fx<>fy then
54     begin
55      inc(j);
56      fa[fx]:=fy;
57      ans0:=ans0+cost[i];
58      f[j]:=i;
59     end;
60    end;
61   end;
62   if (i=p) and (j<w-1) then ans0:=-1;
63  end;
64 
65  procedure main;
66  var i,ans:longint;
67  begin
68   fillchar(use,sizeof(use),true);
69   ans0:=0;
70   kruskal;
71   ans:=maxlongint;
72   tf:=f;
73   for i:=1 to w-1 do
74   begin
75    use[tf[i]]:=false;
76    ans0:=0;
77    kruskal;
78    if (ans0>-1) and (ans0<ans) then ans:=ans0;
79    use[tf[i]]:=true;
80   end;
81   if ans=maxlongint then ans:=-1;
82   writeln(ans);
83  end;
84 
85 Begin
86 assign(input,'secret.in'); reset(input);
87 assign(output,'secret.out'); rewrite(output);
88 
89  init;
90  main;
91 
92 close(input); close(output);
93 End.

这是kruskal的次小生成树。其实还可以优化的。 只不过懒得了。能过教师机就成了。

by the way

我之前的一遍是有用到 并查集来优化 kruskal 的

View Code 
 1 procedure union(x,y:longint);
 2 begin
 3  if r[y]<r[x] then
 4  begin
 5   y:=y xor x;
 6   x:=y xor x;
 7   y:=y xor x;
 8  end
 9 
10 function find(x:longint):longint;
11 begin
12  if x<>p[x] then p[x]:=find(p[x]);
13  exit(p[x]]);
14 end;
15 
16 function clear:boolean;
17 var i,t:longint;
18 begin
19  clear:=true;
20  t:=find(1);
21  for i:=2 to m do 
22   if find(i)<>t then exit(false);
23 end;
24 
25 procedure solve;
26 var i,k:longint;
27 begin
28  for i:=1 to n do 
29  begin
30   if clear then exit;
31   if find(a[i].f)<>find(a[i].t) then 
32   begin
33    inc(ans,a[i].c);
34    union(find(a[i].f),find(a[i].t)); 
35   end;
36  end;   
37 end;

=======

就是这样。