POJ

Description
Give a tree with n vertices,each edge has a length(positive integer less than 1001).
Define dist(u,v)=The min distance between node u and v.
Give an integer k,for every pair (u,v) of vertices is called valid if and only if dist(u,v) not exceed k.
Write a program that will count how many pairs which are valid for a given tree.

Input
The input contains several test cases. The first line of each test case contains two integers n, k. (n<=10000) The following n-1 lines each contains three integers u,v,l, which means there is an edge between node u and v of length l.
The last test case is followed by two zeros.

Output
For each test case output the answer on a single line.

Sample Input

5 4
1 2 3
1 3 1
1 4 2
3 5 1
0 0

Sample Output

8

题目大意：求树上距离小于k的点对个数

裸题，树的点分治

const
    maxn=10010;
var
    n,k,cut,cuts,ans,num,tot:longint;
    next,last,w:array[0..maxn*2]of longint;
    first,size,a:array[0..maxn]of longint;
    flag:array[0..maxn]of boolean;

function max(x,y:longint):longint;
begin
    if x>y then exit(x);
    exit(y);
end;

procedure dfs1(x:longint);
var
    i,s:longint;
begin
    size[x]:=1;
    i:=first[x];
    flag[x]:=false;
    s:=0;
    while i<>0 do
      begin
        if flag[last[i]] then
        begin
          dfs1(last[i]);
          inc(size[x],size[last[i]]);
          s:=max(s,size[last[i]]);
        end;
        i:=next[i];
      end;
    if max(num-size[x],s)<cuts then
    begin
      cut:=x;
      cuts:=max(num-size[x],s);
    end;
    flag[x]:=true;
end;

procedure swap(var x,y:longint);
var
    t:longint;
begin
    t:=x;x:=y;y:=t;
end;

procedure sort(l,r:longint);
var
    i,j,y:longint;
begin
    i:=l;
    j:=r;
    y:=a[(l+r)>>1];
    repeat
      while a[i]<y do
        inc(i);
      while a[j]>y do
        dec(j);
      if i<=j then
      begin
        swap(a[i],a[j]);
        inc(i);
        dec(j);
      end;
    until i>j;
    if i<r then sort(i,r);
    if j>l then sort(l,j);
end;

procedure dfs2(x,d:longint);
var
    i:longint;
begin
    inc(a[0]);
    a[a[0]]:=d;
    flag[x]:=false;
    i:=first[x];
    while i<>0 do
      begin
        if flag[last[i]] then dfs2(last[i],d+w[i]);
        i:=next[i];
      end;
    flag[x]:=true;
end;

function calc(x,d:longint):longint;
var
    l,r:longint;
begin
    calc:=0;
    a[0]:=0;
    dfs2(x,d);
    sort(1,a[0]);
    l:=a[0];
    for r:=1 to a[0] do
      begin
        while (a[l]+a[r]>k) and (l>0) do
          dec(l);
        if l<r then inc(calc,l)
        else inc(calc,r-1);
      end;
end;

procedure work;
var
    i:longint;
begin
    inc(ans,calc(cut,0));
    flag[cut]:=false;
    i:=first[cut];
    while i<>0 do
      begin
        if flag[last[i]] then dec(ans,calc(last[i],w[i]));
        i:=next[i];
      end;
    i:=first[cut];
    while i<>0 do
      begin
        if flag[last[i]] then
        begin
          dfs1(last[i]);
          num:=last[i];
          cuts:=num;
          cut:=last[i];
          dfs1(last[i]);
          work;
        end;
        i:=next[i];
      end;
end;

procedure insert(x,y,z:longint);
begin
    inc(tot);
    last[tot]:=y;
    next[tot]:=first[x];
    first[x]:=tot;
    w[tot]:=z;
end;

procedure init;
var
    i,x,y,z:longint;
begin
    read(n,k);
    if n=0 then halt;
    tot:=0;
    for i:=1 to n do
      begin
        first[i]:=0;
        flag[i]:=true;
      end;
    ans:=0;
    for i:=1 to n-1 do
      begin
        read(x,y,z);
        insert(x,y,z);
        insert(y,x,z);
      end;
    cut:=0;
    cuts:=n;
    dfs1(1);
    work;
    writeln(ans);
end;

begin
    while true do
      init;
end.