Merge pull request soapyigu#339 from soapyigu/Graph

Graph

Graph/AlienDictionary.swift

@@ -9,68 +9,65 @@
 
 class AlienDictionary {
     func alienOrder(_ words: [String]) -> String {
-        var res = "", queueChars = [Character]()
-        var (indegrees, charToChars) = initGraph(words)
+        var (inDegrees, toChars) = buildGraph(words)
 
-        indegrees.keys.filter { indegrees[$0] == 0 }.forEach { queueChars.append($0) }
+        var queue = inDegrees.keys.filter { inDegrees[$0] == 0 }
+        var res = ""
 
-        while !queueChars.isEmpty {
-            let char = queueChars.removeFirst()
-            res.append(char)
+        while !queue.isEmpty {
+            let char = queue.removeFirst()
 
-            guard let toChars = charToChars[char] else {
-                fatalError("Init Graph Error")
-            }
+            res.append(char)
 
-            for toChar in toChars {
-                guard let indegree = indegrees[toChar] else {
-                    fatalError("Init Graph Error")
-                }
+            for nextChar in toChars[char]! {
+                inDegrees[nextChar]! -= 1
 
-                indegrees[toChar] = indegree - 1
-                if indegree == 1 {
-                    queueChars.append(toChar)
+                if inDegrees[nextChar] == 0 {
+                    queue.append(nextChar)
                 }
             }
         }
 
-        return res.count == indegrees.count ? res : ""
+        return res.count == inDegrees.count ? res : ""
     }
 
-    private func initGraph(_ words: [String]) -> ([Character: Int], [Character: [Character]]) {
-        var indegrees = [Character: Int](), charToChars = [Character: [Character]]()
+    private func buildGraph(_ words: [String]) -> ([Character: Int], [Character: [Character]]) {
+        // init inDegrees and toChars
+        var inDegrees = [Character: Int](), toChars = [Character: [Character]]()
 
-        // init indegress and charToChars
         words.forEach { word in
             word.forEach { char in
-                indegrees[char] = 0
-                charToChars[char] = [Character]() 
-            } 
+                inDegrees[char] = 0
+                toChars[char] = [Character]()
+            }
         }
 
-        // refactor indegress and charToChars based on words
+        // update based on orders
         for i in 0..<words.count - 1 {
-            let currentWord = Array(words[i]), nextWord = Array(words[i + 1])
-            var j = 0
-
-            while j < currentWord.count && j < nextWord.count {
-                let currentChar = currentWord[j], nextChar = nextWord[j]
-
-                if nextChar == currentChar {
-                    j += 1
+            let left = Array(words[i]), right = Array(words[i + 1])
+
+            for j in 0..<min(left.count, right.count) {
+                if left[j] == right[j] {
+
+                    // invalid use case
+                    if j + 1 == right.count && right.count < left.count {
+                        return ([Character: Int](), [Character: [Character]]())
+                    }
+
                     continue
                 }
-                if let toChars = charToChars[currentChar], toChars.contains(nextChar) {
+
+                if toChars[left[j]]!.contains(right[j]) {
                     break
                 }
-
-                indegrees[nextChar, default: 0] += 1
-                charToChars[currentChar, default: [Character]()].append(nextChar)
+                
+                inDegrees[right[j]]! += 1
+                toChars[left[j]]!.append(right[j])
                 break
+
             }
-
         }
-
-        return (indegrees, charToChars)
+        
+        return (inDegrees, toChars)
     }
 }

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
+    }
+}