算法模板——LCA（最近公共祖先）

实现的功能如下——在一个N个点的无环图中，共有N-1条边，M个访问中每次询问两个点的距离

原理——既然N个点，N-1条边，则说明这是一棵树，而且联通。所以以1为根节点DFS建树，然后通过求两点的LCA的方式，先求得最近公共祖先，然后再通过深度来求出两点距离

type
    point=^node;
    node=record
               g:longint;
               next:point;
    end;
const
     maxn=100500;
     maxm=trunc(ln(maxn)/ln(2))+1;
var
   i,j,k,l,m,n:longint;
   a:array[0..maxn] of point;
   c:array[0..maxm,0..maxn] of longint;
   d:array[0..maxn] of longint;
function max(x,y:longint):longint;inline;
          begin
               if x>y then max:=x else max:=y;
          end;
function min(x,y:longint):longint;inline;
         begin
              if x<y then min:=x else min:=y;
         end;
procedure swap(var x,y:longint);inline;
          var z:longint;
          begin
               z:=x;x:=y;y:=z;
          end;
procedure add(x,y:longint);inline;
          var p:point;
          begin
               new(p);p^.g:=y;
               p^.next:=a[x];a[x]:=p;
          end;
procedure dfs(x:longint);inline;
          var p:point;
          begin
               p:=a[x];
               while p<>nil do
                     begin
                          if c[0,p^.g]=0 then
                             begin
                                  c[0,p^.g]:=x;
                                  d[p^.g]:=d[x]+1;
                                  dfs(p^.g);
                             end;
                          p:=p^.next;
                     end;
          end;
function getfat(x,y:longint):longint;inline;
         var i,j,k:longint;
         begin
              i:=0;
              while y>0 do
                    begin
                         if odd(y) then x:=c[i,x];
                         inc(i);y:=y div 2;
                    end;
              exit(x);
         end;
function getcom(x,y:longint):longint;inline;
         var i:longint;
         begin
              if d[x]<d[y] then swap(x,y);
              x:=getfat(x,d[x]-d[y]);
              if x=y then exit(x);
              for i:=maxm downto 0 do
                  begin
                       if c[i,x]<>c[i,y] then
                          begin
                               x:=c[i,x];
                               y:=c[i,y];
                          end;
                  end;
              exit(c[0,x]);
         end;

begin
     readln(n,m);
     for i:=1 to n do a[i]:=nil;
     for i:=1 to n-1 do
         begin
              readln(j,k);
              add(j,k);add(k,j);
         end;
     fillchar(d,sizeof(d),0);
     fillchar(c,sizeof(c),0);
     c[0,1]:=-1;
     dfs(1);
     for i:=1 to maxm do
         for j:=1 to n do
             c[i,j]:=c[i-1,c[i-1,j]];
     for i:=1 to m do
         begin
              readln(j,k);
              l:=getcom(j,k);
              writeln(d[j]+d[k]-d[l]-d[l]);
         end;
     readln;
end.