Bellman—Ford算法思想

 1 {单源最短路径的Bellman-ford算法执行v-1次，每次对每条边进行松弛操作如有负权回路则输出"Error"}
 2  const
 3   maxn=100;
 4   maxe=maxn*(maxn-1)div 2;
 5  type
 6   edge=record
 7          a,b,w  :integer;
 8        end;
 9  var
10   edges :array[1..maxe]of edge;
11   dis   :array[1..maxn]of integer;
12   pre   :array[1..maxn]of integer;
13   e,n,s :integer;
14  procedure init;
15  var
16   i     :integer;
17  begin
18   e:=0;
19   assign(input,‘g.in‘);reset(input);
20   readln(n,s);
21   while not eof do
22     begin
23       inc(e);
24       with edges[e] do readln(a,b,w);
25     end;
26   fillchar(dis,sizeof(dis),$7f);//初始值为无穷大
27   dis[s]:=0;pre[s]:=s;
28  end;
29  procedure relax(u,v,w:integer);
30  begin
31   if dis[u]+w<dis[v] then
32     begin
33       dis[v]:=dis[u]+w;
34       pre[v]:=u;
35     end
36  end;
37  function bellman_ford:boolean;
38  var
39   i,j   :integer;
40  begin
41   for i:=1 to n-1 do
42     for j:=1 to e do
43       with edges[j] do relax(a,b,w);
44   for i:=1 to e do
45     with edges[i] do
46       if dis[a]+w<dis[b] then exit(false);
47   exit(true)
48  end;
49  procedure print_path(i:integer);
50  begin
51   if pre[i]<>s then print_path(pre[i]);
52   write(‘-->‘,i)
53  end;
54  procedure show;
55  var
56   i     :integer;
57  begin
58   for i:=1 to n do
59     begin
60       write(i:3,‘:‘,dis[i]:3,‘:‘,s);
61       print_path(i);
62       writeln
63     end;
64  end;
65  {========main========}
66  begin
67  init;
68  if bellman_ford then show
69   else writeln(‘Error!!‘)
70  end.