1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <fstream>
#include <sstream>
#include <set>
#include <map>
#include <vector>
#include <list>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <string>
#include <queue>
#include <bitset>		//UWAGA - w czasie kompilacji musi byc znany rozmiar wektora - nie mozna go zmienic
#include <cassert>
#include <iomanip>		//do setprecision
#include <ctime>
#include <complex>
#include <chrono>
#include<unordered_map>
using namespace std;

#define FOR(i,b,e) for(int i=(b);i<(e);++i)
#define FORQ(i,b,e) for(int i=(b);i<=(e);++i)
#define FORD(i,b,e) for(int i=(b)-1;i>=(e);--i)
#define REP(x, n) for(int x = 0; x < (n); ++x)
#define ALL(u) (u).begin(),(u).end()

#define ST first
#define ND second
#define PB push_back
#define MP make_pair
#define LL long long
#define ULL unsigned LL
#define LD long double

typedef pair<int, int> PII;

const double pi = 3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342;

const int MOD = 1e9 + 7;
const int MR = 25010;

vector<PII> g[MR];

// ile sciezek konczy sie w danym wierzcholku i mapa do kolejnych pozycji
vector<pair<int,map<int, int>>> trie;

vector<int> path, res;
int k, last;

void go(int pos)
{
	// najpierw idz do dzieci od konca -> bierz maksymalne sciezki najpierw
	for (auto it = trie[pos].second.rbegin(); it != trie[pos].second.rend(); it++)
	{
		path.push_back(it->first);
		go(it->second);
		path.pop_back();
	}

	// zobacz, czy mamy sciezke
	last -= trie[pos].first;
	if (last < k && res.empty())
		res = path;
}

multiset<int> sPath, cp;
void add(int pos)
{
	if (cp.empty())
	{
		// koniec sciezki
		trie[pos].first++;
		return;
	}

	auto it = cp.end(); it--; auto v = *it; cp.erase(it);
	auto itM = trie[pos].second.find(v);
	if (itM == trie[pos].second.end())
	{
		// dodaj nowy node
		trie[pos].second[v] = trie.size();
		trie.push_back(MP(0, map<int, int>()));
		add(trie.size() - 1);
	}
	else
		add(itM->second);
}

int done[MR], cnt, root;
void dfs(int nr)
{
	done[nr] = cnt;
	for (const auto &p : g[nr])
		if (done[p.first] != cnt)
		{
			sPath.insert(p.second);

			// dodawaj sciezki od mniejszych do wiekszych
			if (p.first > root)
			{
				cp = sPath;
				add(0);
			}

			dfs(p.first);

			sPath.erase(sPath.find(p.second));
		}
}

LL modPow(LL a, LL x, LL p) {
	// return a^x mod p
	LL res = 1;
	while (x > 0) {
		if (x & 1) res = (res*a) % p;
		a = (a*a) % p;
		x >>= 1;
	}

	return res;
}

int main()
{
	// dodaj pusty wezel
	trie.push_back(MP(0, map<int, int>()));

	int n;
	scanf("%d%d", &n, &k); last = n * (n - 1) / 2;
	REP(i, n - 1)
	{
		int a, b, p;
		scanf("%d%d%d", &a, &b, &p); a--; b--;
		g[a].push_back(MP(b, p));
		g[b].push_back(MP(a, p));
	}

	FORQ(i, 1, n)
	{
		cnt = i;
		root = i - 1;
		dfs(root);
	}

	go(0);

	// wylicz koszt
	int c = 0;
	for (int r : res)
		c = (c + modPow(n, r, MOD)) % MOD;

	printf("%d\n", c);

	return 0;
}

// FOR GNU C++ use the following pattern:
// Uncomment the code below and change your main into main2
// It does not build in MS C++
// But it does increase the stack size from 256 MB on CF and uses GNU C++

//#include <Processthreadsapi.h>
//#include <iostream>
//#include <Synchapi.h>
//#include <windows.h>
//#include <process.h>
//
//DWORD WINAPI MyThreadFunction(LPVOID lpParam) {
//    main2(nullptr);
//    return 0;
//}
//int main() {
//    auto h = CreateThread(nullptr, 1024 << 20, MyThreadFunction, nullptr, STACK_SIZE_PARAM_IS_A_RESERVATION, nullptr);
//    WaitForSingleObject(h, INFINITE);
//}