# Bellman Ford

* Computes shortest paths from **a single source vertex** to all of the other vertices in a weighted directed graph.
* Slower than Dijkstra's algorithm. Its time complexity is `O(VE)`.
* Can handle graph with negative weight edges.
* Works better (than Dijkstra's) for distributed system. Unlike Dijkstra's where we need to find minimum value of all vertices, Bellman-Force considers edges one by one.

## Implementation

Let `dist[u]` be the length of the shortest path from `src` to `u`.

Initially `dist[src] = 0` and `dist[u] = INF` for all other vertices.

Repeat `V - 1` times (since the path in the graph is at most of length `V - 1`):

* For each edge `E = (u, v, weight)`, try to use `E` to update the `dist[v]`: If `dist[u] + weight < dist[v]`, then `dist[v] = dist[u] + weight`.

```cpp
// Time: O(VE)
// Space: O(V)
vector<int> bellmanFord(vector<vector<int>>& edges, int V, int src) {
    vector<int> dist(N, INT_MAX);
    dist[src] = 0;
    for (int i = 1; i < V; ++i) { // try to use all the edges to relax V-1 times.
        for (auto &e : edges) {
            int u = e[0], v = e[1], w = e[2];
            if (dist[u] == INT_MAX) continue;
            dist[v] = min(dist[v], dist[u] + w); // Try to use this edge to relax the cost of `v`.
        }
    }
    return dist;
}
```

## Problems

* [743. Network Delay Time](https://leetcode.com/problems/network-delay-time/)
* [787. Cheapest Flights Within K Stops (Medium)](https://leetcode.com/problems/cheapest-flights-within-k-stops)

## Reference

* [Bellman–Ford algorithm](https://en.wikipedia.org/wiki/Bellman%E2%80%93Ford_algorithm)


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