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Product of minimum edge weight between all pairs of a Tree
  • Difficulty Level : Hard
  • Last Updated : 18 Feb, 2020

Given a tree with N vertices and N-1 Edges. Let’s define a function F(a, b) which is equal to the minimum edge weight in the path between node a & b. The task is to calculate the product of all such F(a, b). Here a&b are unordered pairs and a!=b.

So, basically, we need to find the value of:

                         \prod_{i, j}^nF(i, j)   where 0<=i<j<=n-1.

In the input, we will be given the value of N and then N-1 lines follow. Each line contains 3 integers u, v, w denoting edge between node u and v and it’s weight w. Since the product will be very large, output it modulo 10^9+7.
Examples:

Input :
N = 4
1 2 1
1 3 3
4 3 2
Output : 12
Given tree is:
          1
      (1)/  \(3)
       2     3
              \(2)
               4
We will calculate the minimum edge weight between all the pairs:
F(1, 2) = 1         F(2, 3) = 1
F(1, 3) = 3         F(2, 4) = 1
F(1, 4) = 2         F(3, 4) = 2
Product of all F(i, j) = 1*3*2*1*1*2 = 12 mod (10^9 +7) =12

Input :
N = 5
1 2 1
1 3 3
4 3 2
1 5 4
Output :
288

If we observe carefully then we will see that if there is a set of nodes in which minimum edge weight is w and if we add a node to this set that connects the node with the whole set by an edge of weight W such that W<w then path formed between recently added node to all nodes present in the set will have minimum weight W.
So, here we can apply Disjoint-Set Union concept to solve the problem.
First, sort the data structure according to decreasing weight. Initially assign all nodes as a single set. Now when we merge two sets then do the following:-

      Product=(present weight)^(size of set1*size of set2).                    

We will multiply this product value for all edges of the tree.



Below is the implementation of the above approach:

C++




// C++ Implementation of above approach
#include <bits/stdc++.h>
using namespace std;
#define mod 1000000007
  
// Function to return  (x^y) mod p
int power(int x, unsigned int y, int p)
{
    int res = 1;
  
    x = x % p;
  
    while (y > 0) {
  
        if (y & 1)
            res = (res * x) % p;
        y = y >> 1;
        x = (x * x) % p;
    }
    return res;
}
  
// Declaring size array globally
int size[300005];
int freq[300004];
vector<pair<int, pair<int, int> > > edges;
  
// Initializing DSU data structure
void initialize(int Arr[], int N)
{
    for (int i = 0; i < N; i++) {
        Arr[i] = i;
        size[i] = 1;
    }
}
  
// Function to find the root of ith
// node in the disjoint set
int root(int Arr[], int i)
{
    while (Arr[i] != i) {
        i = Arr[i];
    }
    return i;
}
  
// Weighted union using Path Compression
void weighted_union(int Arr[],
                    int size[], int A, int B)
{
    int root_A = root(Arr, A);
    int root_B = root(Arr, B);
  
    // size of set A is small than size of set B
    if (size[root_A] < size[root_B]) {
        Arr[root_A] = Arr[root_B];
        size[root_B] += size[root_A];
    }
  
    // size of set B is small than size of set A
    else {
        Arr[root_B] = Arr[root_A];
        size[root_A] += size[root_B];
    }
}
  
// Function to add an edge in the tree
void AddEdge(int a, int b, int w)
{
    edges.push_back({ w, { a, b } });
}
  
// Bulid the tree
void MakeTree()
{
    AddEdge(1, 2, 1);
    AddEdge(1, 3, 3);
    AddEdge(3, 4, 2);
}
  
// Function to return the required product
int MinProduct()
{
    int result = 1;
  
    // Sorting the edges with respect to its weight
    sort(edges.begin(), edges.end());
  
    // Start iterating in decreasing order of weight
    for (int i = edges.size() - 1; i >= 0; i--) {
  
        // Determine Curret edge values
        int curr_weight = edges[i].first;
        int Node1 = edges[i].second.first;
        int Node2 = edges[i].second.second;
  
        // Calculate root of each node
        // and size of each set
        int Root1 = root(freq, Node1);
        int Set1_size = size[Root1];
        int Root2 = root(freq, Node2);
        int Set2_size = size[Root2];
  
        // Using the formula
        int prod = Set1_size * Set2_size;
        int Product = power(curr_weight, prod, mod);
  
        // Calculating final result
        result = ((result % mod) * 
                             (Product % mod)) % mod;
  
        // Weighted union using Path Compression
        weighted_union(freq, size, Node1, Node2);
    }
    return result % mod;
}
  
// Driver code
int main()
{
    int n = 4;
  
    initialize(freq, n);
  
    MakeTree();
  
    cout << MinProduct();
}

Python3




# Python3 implementation of the approach
mod = 1000000007
  
# Function to return (x^y) mod p
def power(x: int, y: int, p: int) -> int:
    res = 1
    x %= p
    while y > 0:
        if y & 1:
            res = (res * x) % p
        y = y // 2
        x = (x * x) % p
    return res
  
# Declaring size array globally
size = [0] * 300005
freq = [0] * 300004
edges = []
  
# Initializing DSU data structure
def initialize(arr: list, N: int):
    for i in range(N):
        arr[i] = i
        size[i] = 1
  
# Function to find the root of ith
# node in the disjoint set
def root(arr: list, i: int) -> int:
    while arr[i] != i:
        i = arr[i]
    return i
  
# Weighted union using Path Compression
def weighted_union(arr: list, size: list, A: int, B: int):
    root_A = root(arr, A)
    root_B = root(arr, B)
  
    # size of set A is small than size of set B
    if size[root_A] < size[root_B]:
        arr[root_A] = arr[root_B]
        size[root_B] += size[root_A]
  
    # size of set B is small than size of set A
    else:
        arr[root_B] = arr[root_A]
        size[root_A] += size[root_B]
  
# Function to add an edge in the tree
def AddEdge(a: int, b: int, w: int):
    edges.append((w, (a, b)))
  
# Bulid the tree
def makeTree():
    AddEdge(1, 2, 1)
    AddEdge(1, 3, 3)
    AddEdge(3, 4, 2)
  
# Function to return the required product
def minProduct() -> int:
    result = 1
  
    # Sorting the edges with respect to its weight
    edges.sort(key = lambda a: a[0])
  
    # Start iterating in decreasing order of weight
    for i in range(len(edges) - 1, -1, -1):
  
        # Determine Curret edge values
        curr_weight = edges[i][0]
        node1 = edges[i][1][0]
        node2 = edges[i][1][1]
  
        # Calculate root of each node
        # and size of each set
        root1 = root(freq, node1)
        set1_size = size[root1]
        root2 = root(freq, node2)
        set2_size = size[root2]
  
        # Using the formula
        prod = set1_size * set2_size
        product = power(curr_weight, prod, mod)
  
        # Calculating final result
        result = ((result % mod) * (product % mod)) % mod
  
        # Weighted union using Path Compression
        weighted_union(freq, size, node1, node2)
  
    return result % mod
  
# Driver Code
if __name__ == "__main__":
  
    # Number of nodes and edges
    n = 4
    initialize(freq, n)
    makeTree()
    print(minProduct())
  
# This code is contributed by
# sanjeev2552
Output:
12

Time Complexity : O(N*logN)

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