cs: add «2045. Second Minimum Time to Reach Destination»

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

cs/second-minimum-time-to-reach-destination.cs

@@ -0,0 +1,64 @@
+public class Solution {
+    private TVal Get<TKey, TVal>(Dictionary<TKey, TVal> d, TKey key, Func<TKey, TVal> constructor) {
+        if (!d.ContainsKey(key)) {
+            d[key] = constructor(key);
+        }
+        return d[key];
+    }
+
+    private Dictionary<int, List<int>> GetGraph(int[][] edges) {
+        var g = new Dictionary<int, List<int>>();
+
+        foreach (int[] edge in edges) {
+            var (u, v) = (edge[0], edge[1]);
+            Get(g, u, _ => new List<int>()).Add(v);
+            Get(g, v, _ => new List<int>()).Add(u);
+        }
+
+        return g;
+    }
+
+    private record BFSEntry(int Node, int Index);
+
+    public int SecondMinimum(int n, int[][] edges, int time, int change) {
+        var graph = GetGraph(edges);
+
+        var distances = new int[n + 1, 2];
+        for (var i = 1; i <= n; ++i) {
+            distances[i, 0] = distances[i, 1] = -1;
+        }
+
+        var q = new Queue<BFSEntry>();
+        q.Enqueue(new BFSEntry(1, 0));
+        distances[1, 0] = 0;
+
+        while (q.TryDequeue(out var entry)) {
+            var spentTime = distances[entry.Node, entry.Index];
+            if ((spentTime / change) % 2 == 1) {
+                spentTime = time + change * (spentTime / change + 1);
+            } else {
+                spentTime += time;
+            }
+
+            if (!graph.ContainsKey(entry.Node)) {
+                continue;
+            }
+
+            foreach (var node in graph[entry.Node]) {
+                if (distances[node, 0] == -1) {
+                    distances[node, 0] = spentTime;
+                    q.Enqueue(new BFSEntry(node, 0));
+                } else if (distances[node, 1] == -1 && distances[node, 0] != spentTime) {
+                    if (node == n) {
+                        return spentTime;
+                    }
+
+                    distances[node, 1] = spentTime;
+                    q.Enqueue(new BFSEntry(node, 1));
+                }
+            }
+        }
+
+        return 0;
+    }
+}