[Swift]LeetCode818. 賽車 | Race Car

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Your car starts at position 0 and speed +1 on an infinite number line.  (Your car can go into negative positions.)

Your car drives automatically according to a sequence of instructions A (accelerate) and R (reverse).

When you get an instruction "A", your car does the following: position += speed, speed *= 2.

When you get an instruction "R", your car does the following: if your speed is positive then speed = -1 , otherwise speed = 1.  (Your position stays the same.)

For example, after commands "AAR", your car goes to positions 0->1->3->3, and your speed goes to 1->2->4->-1.

Now for some target position, say the lengthof the shortest sequence of instructions to get there.

Example 1:
Input: 
target = 3
Output: 2
Explanation: 
The shortest instruction sequence is "AA".
Your position goes from 0->1->3.

Example 2:
Input: 
target = 6
Output: 5
Explanation: 
The shortest instruction sequence is "AAARA".
Your position goes from 0->1->3->7->7->6. 

Note:

• 1 <= target <= 10000.

---

你的賽車起始停留在位置 0，速度爲 +1，正行駛在一個無限長的數軸上。（車也能夠向負數方向行駛。）

你的車會根據一系列由 A（加速）和 R（倒車）組成的指令進行自動駕駛 。

當車獲得指令 "A" 時, 將會作出如下操做： position += speed, speed *= 2。

當車獲得指令 "R" 時, 將會作出如下操做：若是當前速度是正數，則將車速調整爲 speed = -1 ；不然將車速調整爲 speed = 1。  (當前所處位置不變。)

例如，當獲得一系列指令 "AAR" 後, 你的車將會走過位置 0->1->3->3，而且速度變化爲 1->2->4->-1。

如今給定一個目標位置，請給出可以到達目標位置的最短指令列表的長度。

示例 1:
輸入: 
target = 3
輸出: 2
解釋: 
最短指令列表爲 "AA"
位置變化爲 0->1->3

示例 2:
輸入: 
target = 6
輸出: 5
解釋: 
最短指令列表爲 "AAARA"
位置變化爲 0->1->3->7->7->6

說明:

• 1 <= target（目標位置） <= 10000。

---

Runtime: 1992 ms

Memory Usage: 33.9 MB

 1 class Solution {
 2     func racecar(_ target: Int) -> Int {
 3         var res:Int = 0
 4         var q:[(Int,Int)] = [(0, 1)]
 5         var visited:Set<String> = ["0,1"]
 6         while(!q.isEmpty)
 7         {
 8             for i in stride(from:q.count,to:0,by:-1)
 9             {
10                 var ele = q.removeFirst()
11                 var pos:Int = ele.0
12                 var speed:Int = ele.1
13                 if pos == target {return res}
14                 var newPos:Int = pos + speed
15                 var newSpeed:Int = speed * 2
16                 var key:String = String(newPos) + "," + String(newSpeed)
17                 if !visited.contains(key) && newPos > 0 && newPos < (target * 2)
18                 {
19                     visited.insert(key)
20                     q.append((newPos, newSpeed))
21                 }
22                 newPos = pos
23                 newSpeed = (speed > 0) ? -1 : 1
24                 key = String(newPos) + "," + String(newSpeed)
25                 if !visited.contains(key) && newPos > 0 && newPos < (target * 2)
26                 {
27                     visited.insert(key)
28                     q.append((newPos, newSpeed))                    
29                 }
30             }
31             res += 1
32         }
33         return -1
34     }
35 }

---

2312ms

 1 import Foundation
 2 class Solution {
 3 
 4     func racecar(_ target: Int) -> Int {
 5         var queue = Queue<(Int, Int)>()
 6         queue.enqueue((0,1))
 7         var visited = Set<String>()
 8         visited.insert("0_1")
 9         var level = 0
10         while !queue.isEmpty {
11             for i in 0...queue.count - 1 {
12                 let cur = queue.dequeue()!
13                 if cur.0 == target {
14                     return level
15                 }
16                 let next1 = (cur.0 + cur.1, cur.1 << 1)
17                 let key1 = "\(next1.0)_\(next1.1)"
18 
19                 if !visited.contains(key1) && 0 < next1.0 && next1.0 < target << 1 {
20                     queue.enqueue(next1)
21                     visited.insert(key1)
22                 }
23 
24                 let next2 = (cur.0, cur.1 > 0 ? -1 : 1)
25                 let key2 = "\(next2.0)_\(next2.1)"
26                 if !visited.contains(key2) && 0 < next2.0 && next2.0 < target << 1 {
27                     queue.enqueue(next2)
28                     visited.insert(key2)
29                 }
30             }
31             level += 1
32         }
33         return -1
34     }
35 }
36 
37 public struct Queue<T> {
38     fileprivate var array = [T]()
39     
40     public var isEmpty: Bool {
41         return array.isEmpty
42     }
43     
44     public var count: Int {
45         return array.count
46     }
47     
48     public mutating func enqueue(_ element: T) {
49         array.append(element)
50     }
51     
52     public mutating func dequeue() -> T? {
53         if isEmpty {
54             return nil
55         } else {
56             return array.removeFirst()
57         }
58     }
59     
60     public var front: T? {
61         return array.first
62     }
63     
64 }