Bellman-Ford Algorithm

Definition:​

The Bellman-Ford Algorithm is a graph algorithm used to find the shortest paths from a single source vertex to all other vertices in a weighted directed graph. Unlike Dijkstra's Algorithm, it can handle graphs with negative edge weights and is also capable of detecting negative weight cycles.

Video Explanation​

Watch

Characteristics:​

• Edge Relaxation Based: The algorithm repeatedly relaxes all edges up to V - 1 times (where V is the number of vertices), progressively finding shorter paths.
• Handles Negative Weights: Works correctly on graphs with negative edge weights, which Dijkstra's Algorithm cannot handle.
• Negative Cycle Detection: After V - 1 iterations, if any edge can still be relaxed, a negative weight cycle exists in the graph.
• Dynamic Programming Approach: Builds up shortest path estimates step by step, ensuring correctness after each round of relaxation.

Time Complexity:​

• Best Case: O(E) When an early-termination optimization is applied and no relaxation occurs after the very first pass — meaning all edges are already in shortest-path order. The algorithm exits after a single round over all E edges.
• Average Case: O(VE) In practice, several rounds of relaxation are needed before distances stabilize. The algorithm performs roughly k passes (where k < V - 1) over all E edges before no updates are found.
• Worst Case: O(VE) When the shortest path to the farthest vertex requires all V - 1 relaxation rounds — for example, in a linear chain graph where the source is at one end. Here V is the number of vertices and E is the number of edges.

Space Complexity:​

• Space Complexity: O(V) Only a distance array of size V is needed to store the shortest path estimates.

Approach:​

The algorithm follows these steps:

1. Initialization:

   • Set the distance to the source vertex as 0.
   • Set the distance to all other vertices as infinity (INF).

2. Relaxation (V - 1 iterations):

   • For each edge (u, v, w), if distance[u] + w < distance[v], update distance[v] = distance[u] + w.
   • Repeat this for all edges, V - 1 times.

3. Negative Cycle Detection:

   • After V - 1 iterations, perform one more pass over all edges.
   • If any distance can still be reduced, the graph contains a negative weight cycle.

4. Output:

   • The distance[] array holds the shortest distance from the source to every other vertex.

Dry Run Example​

Consider the following graph:

Source Vertex: A

Note: For this dry run, we assume the edges are relaxed in the following order: (A, B), (A, C), (C, B), (B, D), (C, E), (D, E).

Initial Distances​

After Pass 1​

After Pass 2​

No further updates occur.

Final Shortest Distances​

The shortest path to B becomes:

A → C → B

with cost:

2 + (-1) = 1

Visualization of Relaxation Process​

Bellman-Ford works by repeatedly relaxing every edge in the graph. Each relaxation attempts to improve the currently known shortest distance to a vertex.

For every edge:

If distance[u] + weight < distance[v]

Then

distance[v] = distance[u] + weight

Relaxation Flow​

Negative Cycle Detection​

Cycle Weight:

-1 + (-2) + 1 = -2

Since the total cycle weight is negative, repeatedly traversing this cycle keeps reducing the path cost. Bellman-Ford detects this during the additional relaxation pass.

After performing V−1 iterations:

One more iteration is performed.

If any distance still decreases:

Graph contains a Negative Weight Cycle

Otherwise:

Shortest Paths Found Successfully

C++ Implementation:​

#include <bits/stdc++.h>
using namespace std;

struct Edge {
    int u, v, w;
};

void bellmanFord(int src, int V, vector<Edge>& edges) {
    vector<int> dist(V, INT_MAX);
    dist[src] = 0;

    // Relax all edges V-1 times
    for (int i = 1; i <= V - 1; i++) {
        for (auto& edge : edges) {
            if (dist[edge.u] != INT_MAX && dist[edge.u] + edge.w < dist[edge.v]) {
                dist[edge.v] = dist[edge.u] + edge.w;
            }
        }
    }

    // Check for negative weight cycles
    for (auto& edge : edges) {
        if (dist[edge.u] != INT_MAX && dist[edge.u] + edge.w < dist[edge.v]) {
            cout << "Graph contains a negative weight cycle!" << endl;
            return;
        }
    }

    // Print shortest distances
    cout << "Vertex\tDistance from Source" << endl;
    for (int i = 0; i < V; i++) {
        cout << i << "\t" << dist[i] << endl;
    }
}

int main() {
    int V = 5;
    vector<Edge> edges = {
        {0, 1, -1}, {0, 2, 4},
        {1, 2, 3},  {1, 3, 2},
        {1, 4, 2},  {3, 2, 5},
        {3, 1, 1},  {4, 3, -3}
    };

    bellmanFord(0, V, edges);
    return 0;
}

Output​

Vertex  Distance from Source
0       0
1       -1
2       2
3       -2
4       1

Java Implementation:​

import java.util.*;

public class BellmanFord {

    static class Edge {
        int u, v, w;
        Edge(int u, int v, int w) {
            this.u = u;
            this.v = v;
            this.w = w;
        }
    }

    public static void bellmanFord(int src, int V, List<Edge> edges) {
        int[] dist = new int[V];
        Arrays.fill(dist, Integer.MAX_VALUE);
        dist[src] = 0;

        // Relax all edges V-1 times
        for (int i = 1; i <= V - 1; i++) {
            for (Edge edge : edges) {
                if (dist[edge.u] != Integer.MAX_VALUE && dist[edge.u] + edge.w < dist[edge.v]) {
                    dist[edge.v] = dist[edge.u] + edge.w;
                }
            }
        }

        // Check for negative weight cycles
        for (Edge edge : edges) {
            if (dist[edge.u] != Integer.MAX_VALUE && dist[edge.u] + edge.w < dist[edge.v]) {
                System.out.println("Graph contains a negative weight cycle!");
                return;
            }
        }

        // Print result
        System.out.println("Vertex\tDistance from Source");
        for (int i = 0; i < V; i++) {
            System.out.println(i + "\t" + dist[i]);
        }
    }

    public static void main(String[] args) {
        int V = 5;
        List<Edge> edges = Arrays.asList(
            new Edge(0, 1, -1), new Edge(0, 2, 4),
            new Edge(1, 2, 3),  new Edge(1, 3, 2),
            new Edge(1, 4, 2),  new Edge(3, 2, 5),
            new Edge(3, 1, 1),  new Edge(4, 3, -3)
        );

        bellmanFord(0, V, edges);
    }
}

Python Implementation:​

def bellman_ford(src, V, edges):
    dist = [float('inf')] * V
    dist[src] = 0

    # Relax all edges V-1 times
    for _ in range(V - 1):
        for u, v, w in edges:
            if dist[u] != float('inf') and dist[u] + w < dist[v]:
                dist[v] = dist[u] + w

    # Check for negative weight cycles
    for u, v, w in edges:
        if dist[u] != float('inf') and dist[u] + w < dist[v]:
            print("Graph contains a negative weight cycle!")
            return

    print("Vertex\tDistance from Source")
    for i in range(V):
        print(f"{i}\t{dist[i]}")


if __name__ == "__main__":
    V = 5
    edges = [
        (0, 1, -1), (0, 2, 4),
        (1, 2, 3),  (1, 3, 2),
        (1, 4, 2),  (3, 2, 5),
        (3, 1, 1),  (4, 3, -3)
    ]
    bellman_ford(0, V, edges)

Complexity Analysis​

Time Complexity​

• Best Case: O(E) — early termination after a single pass when no relaxation is needed.
• Average Case: O(VE) — several passes are required before distances stabilize.
• Worst Case: O(VE) — all V - 1 rounds are needed (e.g., a linear chain graph). The outer loop runs up to V - 1 times and the inner loop processes all E edges each iteration.

Space Complexity​

• Space Complexity: O(V)
• Only the dist[] array of size V is maintained.

Bellman-Ford vs Dijkstra's Algorithm​

When to Use Bellman-Ford Algorithm​

• Negative Edge Weights: When the graph contains edges with negative weights that would cause Dijkstra's Algorithm to fail.
• Negative Cycle Detection: When you need to verify whether a graph contains a negative weight cycle (common in arbitrage detection in financial systems).
• Network Routing: Used in distance-vector routing protocols like RIP (Routing Information Protocol).
• Single-Source Shortest Path: When you need shortest distances from one source to all other nodes.

Assumptions​

• The graph is directed and may contain negative edge weights.
• The algorithm does not work correctly if a negative weight cycle is reachable from the source (it reports the cycle instead).
• If no negative cycle is present, the shortest path is guaranteed after V - 1 iterations.

Conclusion​

The Bellman-Ford Algorithm is a versatile and powerful shortest-path algorithm that extends beyond the limitations of Dijkstra's Algorithm by supporting negative edge weights and detecting negative cycles. While it is slower than Dijkstra's for graphs with non-negative weights, it is the go-to algorithm whenever negative weights or cycle detection are involved, making it indispensable in domains like network routing and financial graph analysis.