Merge pull request #371 from soapyigu/BFS

Add a solution to Bus Routes

Author: soapyigu

BFS/BusRoutes.swift

@@ -0,0 +1,58 @@
+/**
+ * Question Link: https://leetcode.com/problems/bus-routes/
+ * Primary idea: BFS. Build a map for stop and related buses' indexes map. Use a queue to track until the current stop is equal to the target.
+ * 
+ * Time Complexity: O(nm), Space Complexity: O(nm)
+ *
+ */
+
+class BusRoutes {
+    func numBusesToDestination(_ routes: [[Int]], _ source: Int, _ target: Int) -> Int {
+
+        if source == target {
+            return 0
+        }
+
+        let stopBusesMap = buildMap(routes)
+
+        // bfs
+        var queue = [source], res = 0, isVisited = Set<Int>()
+
+        while !queue.isEmpty {
+            let size = queue.count
+
+            for _ in 0..<size {
+                let stop = queue.removeFirst()
+
+                if stop == target {
+                    return res
+                }
+
+                for busIdx in stopBusesMap[stop] ?? []  {
+                    guard !isVisited.contains(busIdx) else {
+                        continue
+                    }
+                    
+                    isVisited.insert(busIdx)
+                    queue.append(contentsOf: routes[busIdx])
+                }
+            }
+            
+            res += 1
+        }
+
+        return -1
+    }
+
+    private func buildMap(_ routes: [[Int]]) -> [Int: Set<Int>] {
+        var stopBusesMap = [Int: Set<Int>]()
+        
+        for (i, bus) in routes.enumerated() {
+            bus.forEach {
+                stopBusesMap[$0, default: Set<Int>()].insert(i)
+            }
+        }
+        
+        return stopBusesMap
+    }
+}