bzoj 1787 lca

来源：互联网发布：淘宝网水兵裤子编辑：程序博客网时间：2024/04/29 21:51

三个点，两两找lca，最后结果一定在这三个点中出，都算出来比较一下就可以了（两个点的最短距离+第三个点到lca的距离）

脑残：建图的时候一定要乖乖的建，不要建成n-1条有向边，会不联通的 orz orz orz

var        n,m,ans1,root,pos:longint;        l,a,b,c,ans2,tt,t:longint;        ta,tb,tc         :longint;        last,d           :array[0..500010] of longint;        pre,other        :array[0..500010] of longint;        vis              :array[0..500010] of boolean;        jump             :array[0..500010,0..20] of longint;        i,j              :longint;procedure connect(x,y:longint);begin   inc(l);   pre[l]:=last[x];   last[x]:=l;   other[l]:=y;end; procedure swap(var a,b:longint);var        c:longint;begin   c:=a;a:=b;b:=c;end; procedure dfs(x:longint);var        p,q:longint;begin   q:=last[x];   while (q<>0) do   begin      p:=other[q];      if not vis[p] then      begin         vis[p]:=true;         d[p]:=d[x]+1;         jump[p,0]:=x;         dfs(p);      end;      q:=pre[q];   end;end; function lca(x,y:longint):longint;var        i,j:longint;        t:longint;begin   t:=0;   if (d[x]>d[y]) then swap(x,y);   //   for j:=0 to 19 do    if ((1<<j) and (d[y]-d[x])<>0) then    begin       inc(t,1<<j);       y:=jump[y,j];    end;    //   if (x<>y) then   begin      for j:=19 downto 0 do       if (jump[x,j]<>jump[y,j]) then       begin          inc(t,1<<j);inc(t,1<<j);          y:=jump[y,j];          x:=jump[x,j];       end;       inc(t,2);       pos:=jump[x,0];   end else pos:=x;   //   exit(t);end; begin   read(n,m);   for i:=1 to n-1 do   begin      read(a,b);      connect(a,b);      vis[b]:=true;   end;   //   for i:=1 to n do if not vis[i] then break;   root:=i;   for i:=1 to n do vis[i]:=false;   d[root]:=1;vis[root]:=true;   dfs(root);   //   for j:=1 to 19 do    for i:=1 to n do jump[i,j]:=jump[jump[i,j-1],j-1];   //   for i:=1 to m do   begin      read(a,b,c);      ans1:=maxlongint;      //      tt:=lca(a,b);      t:=pos;      tt:=tt+lca(pos,c);      if (tt<ans1) then      begin         ans1:=tt;ans2:=t;      end;      //      tt:=lca(a,c);      t:=pos;      tt:=tt+lca(pos,b);      if (tt<ans1) then      begin         ans1:=tt;ans2:=t;      end;      //      tt:=lca(b,c);      t:=pos;      tt:=tt+lca(pos,a);      if (tt<ans1) then      begin         ans1:=tt;ans2:=t;      end;      //      writeln(ans2,' ',ans1);   end;end.

——by Eirlys

0 0