2976. Minimum Cost to Convert String I

Author: sergeyleschev

2501-3000/2976. Minimum Cost to Convert String I.swift

@@ -0,0 +1,73 @@
+class Solution {
+
+    // Solution by Sergey Leschev
+    // 2976. Minimum Cost to Convert String I
+    // BFS/Djikstra
+
+    func minimumCost(
+        _ source: String, _ target: String, _ original: [Character], _ changed: [Character],
+        _ cost: [Int]
+    ) -> Int {
+        var graph = [Character: [(Character, Int)]]()
+
+        // Initializing the graph in the node schema: {parent, {(child, cost)}}
+        for i in 0..<original.count {
+            if graph[original[i]] == nil {
+                graph[original[i]] = [(changed[i], cost[i])]
+            } else {
+                graph[original[i]]!.append((changed[i], cost[i]))
+            }
+        }
+
+        // Distance array required will only need 26*26 space.
+        var distances = Array(repeating: Array(repeating: Int.max, count: 26), count: 26)
+
+        // Running BFS from every node of the original string.
+        for char in original {
+            bfs(graph, char, &distances)
+        }
+
+        var ans = 0
+        for i in source.indices {
+            // No need to add anything to the answer if source and target are the same.
+            if source[i] == target[i] {
+                continue
+            }
+
+            // If the distance is infinite, the target is not achievable, and hence return -1.
+            if distances[Int(source[i].asciiValue! - Character("a").asciiValue!)][
+                Int(target[i].asciiValue! - Character("a").asciiValue!)] == Int.max
+            {
+                return -1
+            }
+
+            // Otherwise, add the corresponding value in the distances array.
+            ans +=
+                distances[Int(source[i].asciiValue! - Character("a").asciiValue!)][
+                    Int(target[i].asciiValue! - Character("a").asciiValue!)]
+        }
+
+        return ans
+    }
+
+    private func bfs(
+        _ graph: [Character: [(Character, Int)]], _ source: Character, _ distances: inout [[Int]]
+    ) {
+        var queue = [(Character, Int)]()
+        queue.append((source, 0))
+
+        while !queue.isEmpty {
+            let (node, distance) = queue.removeFirst()
+
+            for child in graph[node] ?? [] {
+                if distances[Int(source.asciiValue! - Character("a").asciiValue!)][
+                    Int(child.0.asciiValue! - Character("a").asciiValue!)] > distance + child.1
+                {
+                    distances[Int(source.asciiValue! - Character("a").asciiValue!)][
+                        Int(child.0.asciiValue! - Character("a").asciiValue!)] = distance + child.1
+                    queue.append((child.0, distance + child.1))
+                }
+            }
+        }
+    }
+}