LeetCode/cs/find-the-city-with-the-smallest-number-of-neighbors-at-a-threshold-distance.cs

public class Solution {
    private readonly int UNREACHABLE = 1_000_000_007;

    private int[,] MakeDistances(int n, int[][] edges) {
        var distances = new int[n, n];

        // Everything's unreachable at the beginning, except for self-loops
        for (var i = 0; i < n; ++i) {
            for (var j = 0; j < n; ++j) {
                distances[i, j] = UNREACHABLE;
            }

            distances[i, i] = 0;
        }

        // Process the edges
        foreach (int[] edge in edges) {
            var (u, v, weight) = (edge[0], edge[1], edge[2]);

            distances[u, v] = weight;
            distances[v, u] = weight;
        }

        return distances;
    }

    private void FloydWarshall(int n, int[,] distances) {
        for (int k = 0; k < n; ++k) {
            for (int i = 0; i < n; ++i) {
                for (int j = 0; j < n; ++j) {
                    distances[i, j] = Math.Min(
                        distances[i, j],
                        distances[i, k] + distances[k, j]
                    );
                }
            }
        }
    }

    private int FindWithFewestReachable(int n, int[,] distances, int threshold) {
        int foundCity = -1;
        int foundFewest = n;

        for (int i = 0; i < n; ++i) {
            int reachable = 0;
            for (int j = 0; j < n; ++j) {
                if (i == j) {
                    continue;
                }

                if (distances[i, j] <= threshold) {
                    ++reachable;
                }
            }

            if (reachable <= foundFewest) {
                (foundCity, foundFewest) = (i, reachable);
            }
        }

        return foundCity;
    }

    public int FindTheCity(int n, int[][] edges, int distanceThreshold) {
        var distances = MakeDistances(n, edges);
        FloydWarshall(n, distances);
        return FindWithFewestReachable(n, distances, distanceThreshold);
    }
}