bzoj2125 3047

仙人掌上的最短路，这里有详细的题解
http://pan.baidu.com/s/1wzCpC
我觉得讲的很清楚，不懂的见代码注释吧

 type node=record
        po,next,num:longint;
      end;

 var e,w:array[..] of node;
     dfn,low,p,q,mark,s,d,dis,dep,fa:array[..] of longint;
     anc:array[..,..] of longint;
     v:array[..] of boolean;
     st:array[..] of longint;
     t,h,i,len,x,y,z,n,m,tot,qq:longint;

 function min(a,b:longint):longint;
   begin
     if a>b then exit(b) else exit(a);
   end;

 procedure add(x,y,z:longint);
   begin
     inc(len);
     e[len].po:=y;
     e[len].next:=p[x];
     e[len].num:=z;
     p[x]:=len;
   end;

 procedure fadd(x,y:longint);
   begin
     inc(len);
     w[len].po:=y;
     w[len].next:=q[x];
     q[x]:=len;
   end;

 procedure spfa;
   var f,r,i,x,y:longint;
   begin
     f:=;
     r:=;
     st[]:=;
     for i:= to n do  //d[]表示根节点到其他节点的最短路
       d[i]:=;
     while f<=r do
     begin
       x:=st[f];
       i:=p[x];
       v[x]:=false;
       while i<> do
       begin
         y:=e[i].po;
         if d[y]>d[x]+e[i].num then
         begin
           d[y]:=d[x]+e[i].num;
           if not v[y] then
           begin
             inc(r);
             st[r]:=y;
             v[y]:=true;
           end;
         end;
         i:=e[i].next;
       end;
       inc(f);
     end;
   end;

 procedure get(x,y:longint);
   begin
     inc(tot);
     while y<>x do
     begin
       mark[y]:=tot;  //环上节点（除最高点）标为同一颜色
       fadd(x,y);    //建“杨天”树
       y:=fa[y];
     end;
   end;

 procedure dfs1(x:longint);
   var i,y:longint;
   begin
     inc(h);
     dfn[x]:=h;
     low[x]:=h;
     i:=p[x];
     while i<> do
     begin
       y:=e[i].po;
       if fa[x]<>y then
       begin
         if dfn[y]= then
         begin
           fa[y]:=x;
           dis[y]:=dis[x]+e[i].num;  //dis[]表示得到的dfs树上根节点到节点的距离
           dfs1(y);
         end;
         low[x]:=min(low[x],low[y]);
         if low[y]>dfn[x] then
           fadd(x,y);   //dfs树的树边
       end;
       i:=e[i].next;
     end;
     i:=p[x];
     while i<> do
     begin
       y:=e[i].po;
       if (dfn[y]>dfn[x]) and (fa[y]<>x) then
       begin
         get(x,y);  //处理环
         s[tot]:=dis[y]-dis[x]+e[i].num;  //求这个环的长度
       end;
       i:=e[i].next;
     end;
   end;

 procedure dfs2(x:longint);
   var i,y:longint;
   begin
     for i:= to t do
     begin
       y:=anc[x,i-];
       if y<> then anc[x,i]:=anc[y,i-] else break;
     end;
     i:=q[x];
     while i<> do
     begin
       y:=w[i].po;
       fa[y]:=x;
       anc[y,]:=x;
       dep[y]:=dep[x]+;  //在杨天树上的深度
       dfs2(y);
       i:=w[i].next;
     end;
   end;

 function lca(x,y:longint):longint;
   var i,p,a,b,z:longint;
   begin
     if x=y then exit();  //注意
     if dep[x]<dep[y] then
     begin
       p:=x; x:=y; y:=p;
     end;
     a:=x;
     b:=y;
     p:=trunc(ln(dep[x])/ln());
     if dep[a]>dep[b] then
     begin
       for i:=p downto  do
         if dep[a]- shl i>=dep[b] then a:=anc[a,i];
     end;
     if a=b then exit(d[x]-d[a]);  //如果点a是点x的祖先那么a,x之间最短距离为d[x]-d[a]
     for i:=p downto  do
       if (anc[a,i]<>anc[b,i]) then
       begin
         a:=anc[a,i];
         b:=anc[b,i];
       end;
     z:=fa[a];
     if (mark[a]<>) and (mark[a]=mark[b]) then  //最后提到环上两点要讨论
     begin
       p:=abs(dis[a]-dis[b]);
       exit(d[x]-d[a]+d[y]-d[b]+min(p,s[mark[a]]-p));
     end
     else exit(d[x]+d[y]-*d[z]);  //否则直接当树做
   end;

 begin
   readln(n,m,qq);
   for i:= to m do
   begin
     readln(x,y,z);
     add(x,y,z);
     add(y,x,z);
   end;
   t:=trunc(ln(n)/ln());
   spfa;
   len:=;
   dfs1();
   dfs2();
   for i:= to qq do
   begin
     readln(x,y);
     writeln(lca(x,y));
   end;
 end.