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# Path with smallest product of edges with weight>0

• Last Updated : 13 Aug, 2021

Given a directed graph with N nodes and E edges where the weight of each edge is > 0, also given a source S and a destination D. The task is to find the path with the minimum product of edges from S to D. If there is no path from S to D, then print -1.

Examples:

Input: N = 3, E = 3, Edges = {{{1, 2}, 0.5}, {{1, 3}, 1.9}, {{2, 3}, 3}}, S = 1, and D = 3
Output: 1.5
Explanation:
The shortest path will be 1->2->3
with value 0.5*3 = 1.5

Input: N = 3, E = 3, Edges = {{{1, 2}, 0.5}, {{2, 3}, 0.5}, {{3, 1}, 0.5}}, S = 1, and D = 3
Output: cycle detected

Approach: The idea is to use the bellman ford algorithm. It is because Dijkstra’s algorithm cannot be used here as it works only with non-negative edges. It is because while multiplying values between [0-1), the product keeps decreasing indefinitely and 0 is returned finally.
Moreover, cycles need to be detected because if a cycle exists, the product of this cycle will indefinitely decrease the product to 0 and the product will tend to 0. For, simplicity, we will report such cycles.
The following steps can be followed to compute the result:

1. Initialize an array, dis[] with initial value as ‘inf’ except dis[S] as 1.
2. Run a loop from 1 – N-1. For each edge in the graph:
• dis[edge.second] = min(dis[edge.second], dis[edge.first]*weight(edge))
3. Run another loop for each edge in the graph, if any edge exits with (dis[edge.second] > dis[edge.first]*weight(edge)), then cycle is detected.
4. If dist[d] in infinity, return -1, else return dist[d].

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach.``#include ``using` `namespace` `std;` `double` `inf = std::``    ``numeric_limits<``double``>::infinity();` `// Function to return the smallest``// product of edges``double` `bellman(``int` `s, ``int` `d,``               ``vector,``                           ``double``> >``                   ``ed,``               ``int` `n)``{``    ``// If the source is equal``    ``// to the destination``    ``if` `(s == d)``        ``return` `0;` `    ``// Array to store distances``    ``double` `dis[n + 1];` `    ``// Initialising the array``    ``for` `(``int` `i = 1; i <= n; i++)``        ``dis[i] = inf;``    ``dis[s] = 1;` `    ``// Bellman ford algorithm``    ``for` `(``int` `i = 0; i < n - 1; i++)``        ``for` `(``auto` `it : ed)``            ``dis[it.first.second] = min(dis[it.first.second],``                                       ``dis[it.first.first]``                                           ``* it.second);` `    ``// Loop to detect cycle``    ``for` `(``auto` `it : ed) {``        ``if` `(dis[it.first.second]``            ``> dis[it.first.first] * it.second)``            ``return` `-2;``    ``}` `    ``// Returning final answer``    ``if` `(dis[d] == inf)``        ``return` `-1;``    ``else``        ``return` `dis[d];``}` `// Driver code``int` `main()``{` `    ``int` `n = 3;``    ``vector, ``double``> > ed;` `    ``// Input edges``    ``ed = { { { 1, 2 }, 0.5 },``           ``{ { 1, 3 }, 1.9 },``           ``{ { 2, 3 }, 3 } };` `    ``// Source and Destination``    ``int` `s = 1, d = 3;` `    ``// Bellman ford``    ``double` `get = bellman(s, d, ed, n);` `    ``if` `(get == -2)``        ``cout << ``"Cycle Detected"``;``    ``else``        ``cout << get;``}`

## Java

 `// Java implementation of the approach``import` `java.util.ArrayList;``import` `java.util.Arrays;``import` `java.util.PriorityQueue;` `class` `Pair``{``    ``K first;``    ``V second;` `    ``public` `Pair(K first, V second)``    ``{``        ``this``.first = first;``        ``this``.second = second;``    ``}``}` `class` `GFG{` `static` `final` `float` `inf = Float.POSITIVE_INFINITY;` `// Function to return the smallest``// product of edges``static` `float` `bellman(``int` `s, ``int` `d,``                     ``ArrayList, Float>> ed,``                     ``int` `n)``{``    ` `    ``// If the source is equal``    ``// to the destination``    ``if` `(s == d)``        ``return` `0``;` `    ``// Array to store distances``    ``float``[] dis = ``new` `float``[n + ``1``];` `    ``// Initialising the array``    ``Arrays.fill(dis, inf);``    ``dis[s] = ``1``;` `    ``// Bellman ford algorithm``    ``for``(``int` `i = ``0``; i < n - ``1``; i++)``        ``for``(Pair, Float> it : ed)``            ``dis[it.first.second] = Math.min(dis[it.first.second],``                                            ``dis[it.first.first] *``                                                ``it.second);` `    ``// Loop to detect cycle``    ``for``(Pair, Float> it : ed)``    ``{``        ``if` `(dis[it.first.second] >``            ``dis[it.first.first] *``                ``it.second)``            ``return` `-``2``;``    ``}` `    ``// Returning final answer``    ``if` `(dis[d] == inf)``        ``return` `-``1``;``    ``else``        ``return` `dis[d];``}` `// Driver code``public` `static` `void` `main(String[] args)``{``    ``int` `n = ``3``;``    ` `    ``// Input edges``    ``ArrayList, Float>> ed = ``new` `ArrayList<>(``            ``Arrays.asList(``                      ``new` `Pair, Float>(``                           ``new` `Pair(``1``, ``2``), ``0``.5f),``                    ``new` `Pair, Float>(``                         ``new` `Pair(``1``, ``3``), ``1``.9f),``                    ``new` `Pair, Float>(``                         ``new` `Pair(``2``, ``3``), 3f)));` `    ``// Source and Destination``    ``int` `s = ``1``, d = ``3``;` `    ``// Bellman ford``    ``float` `get = bellman(s, d, ed, n);` `    ``if` `(get == -``2``)``        ``System.out.println(``"Cycle Detected"``);``    ``else``        ``System.out.println(get);``}``}` `// This code is contributed by sanjeev2552`

## Python3

 `# Python3 implementation of the approach.``import` `sys` `inf ``=` `sys.maxsize;` `# Function to return the smallest``# product of edges``def` `bellman(s, d, ed, n) :` `    ``# If the source is equal``    ``# to the destination``    ``if` `(s ``=``=` `d) :``        ``return` `0``;` `    ``# Array to store distances``    ``dis ``=` `[``0``]``*``(n ``+` `1``);` `    ``# Initialising the array``    ``for` `i ``in` `range``(``1``, n ``+` `1``) :``        ``dis[i] ``=` `inf;``        ` `    ``dis[s] ``=` `1``;` `    ``# Bellman ford algorithm``    ``for` `i ``in` `range``(n ``-` `1``) :``        ``for` `it ``in` `ed :``            ``dis[it[``1``]] ``=` `min``(dis[it[``1``]], dis[it[``0``]] ``*` `ed[it]);` `    ``# Loop to detect cycle``    ``for` `it ``in` `ed :``        ``if` `(dis[it[``1``]] > dis[it[``0``]] ``*` `ed[it]) :``            ``return` `-``2``;` `    ``# Returning final answer``    ``if` `(dis[d] ``=``=` `inf) :``        ``return` `-``1``;``    ``else` `:``        ``return` `dis[d];` `# Driver code``if` `__name__ ``=``=` `"__main__"` `:` `    ``n ``=` `3``;``    ` `    ``# Input edges``    ``ed ``=` `{ ( ``1``, ``2` `) : ``0.5` `,``        ``( ``1``, ``3` `) : ``1.9` `,``        ``( ``2``, ``3` `) : ``3` `};` `    ``# Source and Destination``    ``s ``=` `1``; d ``=` `3``;` `    ``# Bellman ford``    ``get ``=` `bellman(s, d, ed, n);` `    ``if` `(get ``=``=` `-``2``) :``        ``print``(``"Cycle Detected"``);``    ``else` `:``        ``print``(get);` `# This code is contributed by AnkitRai01`

## Javascript

 ``
Output:
`1.5`

Time complexity: O(E*V)
Auxiliary Space: O(V).

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