cs: add «2976. Minimum Cost to Convert String I»

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

cs/minimum-cost-to-convert-string-i.cs

@@ -0,0 +1,56 @@
+public class Solution {
+    private readonly int UNREACHABLE = 1_000_000_007;
+    private readonly int LETTERS = 26;
+
+    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[,] ConstructGraph(char[] original, char[] changed, int[] costs) {
+        var distances = new int[LETTERS, LETTERS];
+
+        // Everything's unreachable
+        for (var i = LETTERS * LETTERS - 1; i >= 0; --i) {
+            distances[i / LETTERS, i % LETTERS] = UNREACHABLE;
+        }
+
+        // Go through the possible changes
+        for (var i = 0; i < original.length; ++i) {
+            var (u, v, cost) = (original[i] - 'a', changed[i] - 'a', costs[i]);
+            distances[u, v] = Math.Min(distances[u, v], cost);
+        }
+
+        // Handle self-loops
+        for (var i = 0; i < LETTERS; ++i) {
+            distances[i, i] = 0;
+        }
+
+        return distances;
+    }
+
+    public long MinimumCost(string source, string target, char[] original, char[] changed, int[] cost) {
+        var graph = ConstructGraph(original, changed, cost);
+        FloydWarshall(LETTERS, graph);
+
+        var minCost = 0;
+        for (int i = 0; i < source.Length; ++i) {
+            var operationCost = graph[source[i] - 'a', target[i] - 'a'];
+            if (operationCost == UNREACHABLE) {
+                return -1;
+            }
+
+            minCost += operationCost;
+        }
+
+        return minCost;
+    }
+}