[Swift]LeetCode710. 黑名單中的隨機數 | Random Pick with Blacklist

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➤微信公衆號：山青詠芝（shanqingyongzhi）
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Given a blacklist B containing unique integers from [0, N), write a function to return a uniform random integer from [0, N) which is NOT in B.

Optimize it such that it minimizes the call to system’s Math.random().

Note:

1. 1 <= N <= 1000000000
2. 0 <= B.length < min(100000, N)
3. [0, N) does NOT include N. See interval notation.

Example 1:

Input: 
["Solution","pick","pick","pick"]
[[1,[]],[],[],[]] Output: [null,0,0,0] 

Example 2:

Input: 
["Solution","pick","pick","pick"]
[[2,[]],[],[],[]] Output: [null,1,1,1] 

Example 3:

Input: 
["Solution","pick","pick","pick"]
[[3,[1]],[],[],[]] Output: [null,0,0,2] 

Example 4:

Input: 
["Solution","pick","pick","pick"]
[[4,[2]],[],[],[]] Output: [null,1,3,1] 

Explanation of Input Syntax:

The input is two lists: the subroutines called and their arguments. Solution's constructor has two arguments, N and the blacklist B. pick has no arguments. Arguments are always wrapped with a list, even if there aren't any.

---

給定一個包含 [0，n ) 中獨特的整數的黑名單 B，寫一個函數從 [ 0，n ) 中返回一個不在 B 中的隨機整數。

對它進行優化使其儘可能少調用系統方法 Math.random() 。

提示:

1. 1 <= N <= 1000000000
2. 0 <= B.length < min(100000, N)
3. [0, N) 不包含 N，詳細參見 interval notation 。

示例 1:

輸入: 
["Solution","pick","pick","pick"]
[[1,[]],[],[],[]]
輸出: [null,0,0,0]

示例 2:

輸入: 
["Solution","pick","pick","pick"]
[[2,[]],[],[],[]]
輸出: [null,1,1,1]

示例 3:

輸入: 
["Solution","pick","pick","pick"]
[[3,[1]],[],[],[]]
Output: [null,0,0,2]

示例 4:

輸入: 
["Solution","pick","pick","pick"]
[[4,[2]],[],[],[]]
輸出: [null,1,3,1]

輸入語法說明：

輸入是兩個列表：調用成員函數名和調用的參數。Solution的構造函數有兩個參數，N 和黑名單 B。pick 沒有參數，輸入參數是一個列表，即便參數爲空，也會輸入一個 [] 空列表。

---

Runtime: 984 ms

Memory Usage: 23.4 MB

 1 class Solution {
 2     var M:Int = 0
 3     var map:[Int:Int] = [Int:Int]()
 4     init(_ N: Int, _ blacklist: [Int]) {
 5         var N = N
 6         for b in blacklist
 7         {
 8             map[b] = -1
 9         }
10         M = N - map.count
11         for b in blacklist
12         {
13             if b < M
14             {
15                 while (map[N - 1] != nil)
16                 {
17                     N -= 1
18                 }
19                 map[b] = N - 1
20                 N -= 1
21             }
22         }        
23     }
24     
25     func pick() -> Int {
26         var p:Int = Int.random(in:0..<M)
27         return map[p] == nil ? p : map[p]!      
28     }
29 }
30 
31 /**
32  * Your Solution object will be instantiated and called as such:
33  * let obj = Solution(N, blacklist)
34  * let ret_1: Int = obj.pick()
35  */ 

---

1628ms

 1 class Solution {
 2 
 3     init(_ N: Int, _ blacklist: [Int]) {
 4         M = N - blacklist.count
 5         for n in blacklist {
 6             mapping[n] = 0
 7         }
 8         
 9         var i = M
10         for n in blacklist {
11             if n < M {
12                 while mapping[i] != nil {
13                     i += 1
14                 }
15                 mapping[n] = i
16                 i += 1
17             }
18         }
19     }
20     
21     func pick() -> Int {
22         let r = Int.random(in: 0..<M)
23         if let n = mapping[r] {
24             return n
25         }
26         return r
27     }
28     
29     var M : Int
30     var mapping = [Int: Int]()
31 }
32 
33 /**
34  * Your Solution object will be instantiated and called as such:
35  * let obj = Solution(N, blacklist)
36  * let ret_1: Int = obj.pick()
37  */