Shortest distance between two nodes in Graph by reducing weight of an edge by half

Difficulty Level : Basic
Last Updated : 06 Sep, 2021

Given a weighted undirected graph consisting of N nodes and M edges, the task is to find the shortest distance between two nodes A and B by reducing the weight of one edge by half.

Examples:

Input: A = 0, B = 2, Below is the graph
Output: 8
Explaination:
After reducing the weight of the edge connecting 1 and 2 by half modifies its new weight to 4. Now, the shortest distance to reach 2 from 0 through the path 0 -> 1 -> 2 is (4 + 4) = 8.
Therefore, print 8.

Approach: The given problem can be solved by maintaining two arrays, the shortest distance array taking source node as A which will store the shortest distance of all nodes from A, and similarly the shortest distance array taking source node as B. These arrays can be calculated using Dijkstra’s algorithm. Follow the below steps to solve the above problem:

Use Dijkstra’s algorithm to store the shortest distance of each node from A into an array disA[].
Use Dijkstra’s algorithm to store the shortest distance of each node from B into an array disB[].
Suppose edge_i= {u_i, v_i, wt_i} i.e., edge_i connects node u_i to v_i and has a weight of wt_i.
Now, iterate over all edges and for every edge keep track of the function as:

f(edge_i) = min( disA[u_i] + disB[v_i], disA[v_i] + disB[u_i]) + (wt_i/2).

The above relation gives the minimum value of f(edge), which is the resultant shortest distance.

Below is the implementation of the above approach:

C++14

// C++ program for the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Stores the input Graph
vector<pair<int, int> > graph[100001];
 
// Stores edges of input Graph
vector<vector<int> > edges;
 
// Function to input Edges
void add_edge(int u, int v, int w)
{
    graph[u].push_back({ v, w });
    graph[v].push_back({ u, w });
    edges.push_back({ u, v, w });
}
 
// Function to find the shortest distance
// to each node from the src node using
// Dijkstra’s Algorithm
vector<int> dijsktras(int src, int N)
{
    // Stores the shortest distance of
    // each node form src node
    vector<int> dis(N, INT_MAX);
 
    vector<bool> vis(N, false);
 
    // Stores the node and current
    // minimum distance in a heap
    priority_queue<pair<int, int>,
                   vector<pair<int, int> >,
                   greater<pair<int, int> > >
        pq;
    pq.push({ 0, src });
    dis[src] = 0;
 
    // BFS for single source shortest
    // path algorithm
    while (!pq.empty()) {
 
        // Top of the PQ
        auto cur = pq.top();
        pq.pop();
 
        // Store the node and weight
        int node = cur.second;
        int weight = cur.first;
 
        // If node is already visited
        if (vis[node])
            continue;
        vis[node] = true;
 
        // Traverse the adjacency list
        // of the node
        for (auto child : graph[node]) {
 
            // If the distance obtained
            // from parent is less than
            // the current minimum
            // distance stored for child
            if (dis[child.first]
                > child.second + weight) {
                dis[child.first] = weight
                                   + child.second;
 
                // Push the next pair
                // in the PQ
                pq.push({ dis[child.first],
                          child.first });
            }
        }
    }
 
    // Return the maximum distance
    return dis;
}
 
// Function to find shortest distance
// between two nodes by reducing any
// one weight of an edge by half
int shortestDistance(int N, int M,
                     int A, int B)
{
    // Stores the the shortest distance
    // of each node from A
    vector<int> disA = dijsktras(A, N);
 
    // Stores the the shortest distance
    // of each node from B
    vector<int> disB = dijsktras(B, N);
 
    int ans = disA[B];
    for (auto edge : edges) {
        int u = edge[0], v = edge[1];
        int weight = edge[2];
 
        // Calculate the value of f(edge)
        // for the current edge
        int cur = min(disA[u] + disB[v],
                      disA[v] + disB[u])
                  + (weight / 2);
 
        // Keep track of the minimum of
        // f(edge) for all edges
        ans = min(ans, cur);
    }
 
    // Return Answer
    return ans;
}
 
// Driver Code
int main()
{
    int N = 9, M = 14, A = 0, B = 2;
 
    // Create a Graph
    add_edge(0, 1, 4);
    add_edge(1, 2, 8);
    add_edge(2, 3, 7);
    add_edge(3, 4, 9);
    add_edge(4, 5, 10);
    add_edge(5, 6, 2);
    add_edge(6, 7, 1);
    add_edge(7, 0, 8);
    add_edge(1, 7, 11);
    add_edge(7, 8, 7);
    add_edge(2, 8, 2);
    add_edge(6, 8, 6);
    add_edge(2, 5, 4);
    add_edge(3, 5, 14);
 
    // Function Call
    cout << shortestDistance(N, M, A, B);
 
    return 0;
}

Output:

Time Complexity: O(M*log N)
Auxiliary Space:O(N + M)

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Shortest distance between two nodes in Graph by reducing weight of an edge by half

C++14

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