Add a solution to Race Car

Author: soapyigu

BFS/RaceCar.swift

@@ -0,0 +1,53 @@
+/**
+ * Question Link: https://leetcode.com/problems/race-car/
+ * Primary idea: BFS to go over A or R at a specific step. Only add R to the queue when necessary.
+ * 
+ * Time Complexity: O(nlogn), Space Complexity: O(n)
+ */
+
+class RaceCar {
+    func racecar(_ target: Int) -> Int {
+        let startNode = Node(speed: 1, position: 0)
+        var queue = [startNode], len = 0, isVisited = Set<Node>()
+        
+        while !queue.isEmpty {
+            let size = queue.count
+            
+            for _ in 0..<size {
+                let node = queue.removeFirst()
+                
+                if node.position == target {
+                    return len
+                }
+                
+                if isVisited.contains(node) {
+                    continue
+                }
+                
+                isVisited.insert(node)
+                
+                let ANode = Node(speed: node.speed * 2, position: node.position + node.speed)
+                queue.append(ANode)
+               
+                if shouldRevert(node, target) {
+                    let RNode = Node(speed: node.speed > 0 ? -1 : 1, position: node.position)
+                    queue.append(RNode)
+                }
+                    
+            }
+            
+            len += 1
+        }
+        
+        return len
+    }
+    
+    private func shouldRevert(_ node: Node, _ target: Int) -> Bool {
+        return (node.position + node.speed > target && node.speed > 0) || (node.position + node.speed < target && node.speed < 0)
+    }
+    
+    struct Node: Hashable {
+        let speed: Int
+        let position: Int
+    }
+}