bzoj 2435 BFS

我们可以先将无根树变成有根树，随便选一个点当根就行了，我们选1，bfs求出来每个点 

的size值，代表以它为根节点的子树中有多少个节点，（dfs可能会爆栈），然后再对于每一条

边算就好了

我tle了，也不知道咋会事儿，可能是pascal的int64的运算有些耗时。。。。

 

/**************************************************************
    Problem: 2435
    User: BLADEVIL
    Language: Pascal
    Result: Time_Limit_Exceed
****************************************************************/
 
//By BLADEVIL
var
    n                       :longint;
    pre, other, last, len   :array[0..2000010] of int64;
    l                       :longint;
    flag                    :array[0..1000010] of boolean;
    size, que, dep          :array[0..1000010] of int64;
    ans                     :int64;
    fuse                    :array[0..2000010] of boolean;
     
procedure connect(x,y,z:longint);
begin
    inc(l);
    pre[l]:=last[x];
    last[x]:=l;
    other[l]:=y;
    len[l]:=z;
end;
     
procedure init;
var
    i                       :longint;
    x, y, z                 :longint;
     
begin
    read(n);
    for i:=1 to n-1 do
    begin
        read(x,y,z);
        connect(x,y,z);
        connect(y,x,z);
    end;
end;
 
procedure bfs;
var
    h, t, cur               :longint;
    q, p                    :longint;
begin
    h:=0; t:=1;
    que[1]:=1;
    flag[1]:=true;
    while h<>t do
    begin
        inc(h);
        cur:=que[h];
        q:=last[cur];
        while q<>0 do
        begin
            p:=other[q];
            if not flag[p] then
            begin
                fuse[q]:=true;
                inc(t);
                que[t]:=p;
                dep[p]:=dep[cur]+1;
                flag[p]:=true;
            end;
            q:=pre[q];
        end;
    end;
end;
 
procedure main;
var
    i                       :longint;
    q, p                    :longint;
begin
    bfs;
    for i:=n downto 1 do
    begin
        q:=last[que[i]];
        size[que[i]]:=1;
        while q<>0 do
        begin
            p:=other[q];
            if dep[p]>dep[que[i]] then inc(size[que[i]],size[p]);
            q:=pre[q];
        end;
    end;
    for i:=1 to l do if fuse[i] then ans:=ans+abs(n-2*size[other[i]])*len[i];
    writeln(ans);
end;
 
begin
    init;
    main;
end.