cs: add «1334. Find the City With the Smallest Number of Neighbors at a Threshold Distance»

Signed-off-by: Matej Focko <me@mfocko.xyz>

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

@@ -0,0 +1,69 @@
+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);
+    }
+}