[Swift]LeetCode818. 赛车 | Race Car

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➤微信公众号：山青咏芝（shanqingyongzhi）
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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。

---

Runtime: 1992 ms

Memory Usage: 33.9 MB

class Solution {
    func racecar(_ target: Int) -> Int {
        var res:Int = 0
        var q:[(Int,Int)] = [(0, 1)]
        var visited:Set<String> = ["0,1"]
        while(!q.isEmpty)
        {
            for i in stride(from:q.count,to:0,by:-1)
            {
                var ele = q.removeFirst()
                var pos:Int = ele.0
                var speed:Int = ele.1
                if pos == target {return res}
                var newPos:Int = pos + speed
                var newSpeed:Int = speed * 2
                var key:String = String(newPos) + "," + String(newSpeed)
                if !visited.contains(key) && newPos > 0 && newPos < (target * 2)
                {
                    visited.insert(key)
                    q.append((newPos, newSpeed))
                }
                newPos = pos
                newSpeed = (speed > 0) ? -1 : 1
                key = String(newPos) + "," + String(newSpeed)
                if !visited.contains(key) && newPos > 0 && newPos < (target * 2)
                {
                    visited.insert(key)
                    q.append((newPos, newSpeed))                    
                }
            }
            res += 1
        }
        return -1
    }
}

---

2312ms

import Foundation
class Solution {

    func racecar(_ target: Int) -> Int {
        var queue = Queue<(Int, Int)>()
        queue.enqueue((0,1))
        var visited = Set<String>()
        visited.insert("0_1")
        var level = 0
        while !queue.isEmpty {
            for i in 0...queue.count - 1 {
                let cur = queue.dequeue()!
                if cur.0 == target {
                    return level
                }
                let next1 = (cur.0 + cur.1, cur.1 << 1)
                let key1 = "(next1.0)_(next1.1)"

                if !visited.contains(key1) && 0 < next1.0 && next1.0 < target << 1 {
                    queue.enqueue(next1)
                    visited.insert(key1)
                }

                let next2 = (cur.0, cur.1 > 0 ? -1 : 1)
                let key2 = "(next2.0)_(next2.1)"
                if !visited.contains(key2) && 0 < next2.0 && next2.0 < target << 1 {
                    queue.enqueue(next2)
                    visited.insert(key2)
                }
            }
            level += 1
        }
        return -1
    }
}

public struct Queue<T> {
    fileprivate var array = [T]()
    
    public var isEmpty: Bool {
        return array.isEmpty
    }
    
    public var count: Int {
        return array.count
    }
    
    public mutating func enqueue(_ element: T) {
        array.append(element)
    }
    
    public mutating func dequeue() -> T? {
        if isEmpty {
            return nil
        } else {
            return array.removeFirst()
        }
    }
    
    public var front: T? {
        return array.first
    }
    
}