求一个图的最小生成树，其中某些边不收费（可以这样理解）。
这次用kruscal打的，对于不收费的边先全部加入，再维护即可。
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1 program pku2421(input,output);
2 var
3 father : array[0..200] of longint;
4 f : array[0..200,0..200] of longint;
5 x,y,w : array[0..40000] of longint;
6 n,m,q,answer : longint;
7 procedure init;
8 var
9 i,j : longint;
10 begin
11 readln(n);
12 for i:=1 to n do
13 father[i]:=i;
14 m:=0;
15 for i:=1 to n do
16 begin
17 for j:=1 to n do
18 begin
19 read(f[i,j]);
20 if (i<=j)and(f[i,j]<>0) then
21 begin
22 inc(m);
23 x[m]:=i;
24 y[m]:=j;
25 w[m]:=f[i,j];
26 end;
27 end;
28 readln;
29 end;
30 end; { init }
31 procedure swap(var aa,bb :longint );
32 var
33 tt : longint;
34 begin
35 tt:=aa;
36 aa:=bb;
37 bb:=tt;
38 end; { swap }
39 procedure sort(p,q :longint );
40 var
41 i,j,mm : longint;
42 begin
43 i:=p;
44 j:=q;
45 mm:=w[(i+j)>>1];
46 repeat
47 while w[i]<mm do
48 inc(i);
49 while w[j]>mm do
50 dec(j);
51 if i<=j then
52 begin
53 swap(w[i],w[j]);
54 swap(x[i],x[j]);
55 swap(y[i],y[j]);
56 inc(i);
57 dec(j);
58 end;
59 until i>j;
60 if i<q then sort(i,q);
61 if j>p then sort(p,j);
62 end; { sort }
63 function getfather(x :longint ):longint;
64 begin
65 if father[x]=x then
66 exit(x);
67 father[x]:=getfather(father[x]);
68 exit(father[x]);
69 end; { getfather }
70 function check():boolean;
71 var
72 i : longint;
73 begin
74 for i:=2 to n do
75 if getfather(1)<>getfather(i) then
76 exit(false);
77 exit(true);
78 end; { check }
79 procedure main;
80 var
81 i : longint;
82 xx,yy : longint;
83 begin
84 readln(q);
85 for i:=1 to q do
86 begin
87 readln(xx,yy);
88 xx:=getfather(xx);
89 yy:=getfather(yy);
90 if xx<>yy then
91 father[xx]:=yy;
92 end;
93 for i:=1 to m do
94 begin
95 xx:=getfather(x[i]);
96 yy:=getfather(y[i]);
97 if xx=yy then
98 continue
99 else
100 begin
101 inc(answer,w[i]);
102 father[xx]:=yy;
103 if check then
104 break;
105 end;
106 end;
107 writeln(answer);
108 end; { main }
109 begin
110 init;
111 sort(1,m);
112 main;
113 end.