Add a solution to the Cheapest Flights Within K Stops

Author: soapyigu

Graph/CheapestFlightsKStops.swift

@@ -0,0 +1,52 @@
+/**
+ * Question Link: https://leetcode.com/problems/cheapest-flights-within-k-stops/
+ * Primary idea: Topological sort + BFS. 
+ * Build a graph with each srouce and destination as an edge. Put the source as the head node for the queue, visit every possible destinaiton and update the result with the possible smallest value.
+ *
+ * Time Complexity: O(n * (n - 1) + K), Space Complexity: O(n * (n - 1)), 
+ * n represents number of the cities
+ */
+
+class CheapestFlightsKStops {
+    func findCheapestPrice(_ n: Int, _ flights: [[Int]], _ src: Int, _ dst: Int, _ k: Int) -> Int {
+        var srcDestinations = buildGraph(flights)
+        var queue = [(src, 0)], dstPrice = [Int: Int](), stopCount = 0
+        
+        while !queue.isEmpty && stopCount < k + 1 {
+            
+            let currentQueueLen = queue.count
+            
+            for _ in 0..<currentQueueLen {
+                let (from, fromPrice) = queue.removeFirst()
+                
+                guard let destinations = srcDestinations[from] else {
+                    continue
+                }
+            
+                for (to, price) in destinations {
+                    if let prevPrice = dstPrice[to], prevPrice <= fromPrice + price {
+                        continue
+                    } else {
+                        dstPrice[to] = fromPrice + price
+                        queue.append((to, fromPrice + price))
+                    }
+                }
+            
+            }
+            stopCount += 1
+        }
+        
+        return dstPrice[dst] ?? -1
+    }
+    
+    // src -> (dst, price)
+    private func buildGraph(_ flights: [[Int]]) -> [Int: [(Int, Int)]] {
+        var srcDestinations = [Int: [(Int, Int)]]()
+        
+        for flight in flights {
+            srcDestinations[flight[0], default:[(Int, Int)]()].append((flight[1], flight[2]))
+        }
+                       
+        return srcDestinations
+    }
+}