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:
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1 <= target <= 10000.
你的赛车起始停留在位置 0，速度为 +1，正行驶在一个无限长的数轴上。（车也可以向负数方向行驶。）
你的车会根据一系列由 A（加速）和 R（倒车）组成的指令进行自动驾驶 。
当车得到指令 "A" 时,将会做出以下操作： position += speed,speed *= 2。
当车得到指令 "R" 时,将会做出以下操作：如果当前速度是正数，则将车速调整为 speed = -1 ；否则将车速调整为 speed = 1。 (当前所处位置不变。)
例如，当得到一系列指令 "AAR" 后,你的车将会走过位置 0->1->3->3，并且速度变化为 1->2->4->-1。
现在给定一个目标位置，请给出能够到达目标位置的最短指令列表的长度。
示例 1: 输入: target = 3 输出: 2 解释: 最短指令列表为 "AA" 位置变化为 0->1->3
示例 2: 输入: target = 6 输出: 5 解释: 最短指令列表为 "AAara" 位置变化为 0->1->3->7->7->6
说明:
-
1 <= target（目标位置） <= 10000。
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 }
