[Swift]LeetCode381. O(1) 时间插入、删除和获取随机元素

原文地址：https://www.cnblogs.com/strengthen/p/10282676.html 

Design a data structure that supports all following operations in averageO(1) time.

Note: Duplicate elements are allowed. 

1. insert(val): Inserts an item val to the collection.
2. remove(val): Removes an item val from the collection if present.
3. getRandom: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains. 

Example:

// Init an empty collection.
RandomizedCollection collection = new RandomizedCollection();

// Inserts 1 to the collection. Returns true as the collection did not contain 1.
collection.insert(1);

// Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].
collection.insert(1);

// Inserts 2 to the collection, returns true. Collection now contains [1,1,2].
collection.insert(2);

// getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.
collection.getRandom();

// Removes 1 from the collection, returns true. Collection now contains [1,2].
collection.remove(1);

// getRandom should return 1 and 2 both equally likely.
collection.getRandom();

---

设计一个支持在平均 时间复杂度 O(1) 下， 执行以下操作的数据结构。

注意: 允许出现重复元素。

1. insert(val)：向集合中插入元素 val。
2. remove(val)：当 val 存在时，从集合中移除一个 val。
3. getRandom：从现有集合中随机获取一个元素。每个元素被返回的概率应该与其在集合中的数量呈线性相关。

示例:

// 初始化一个空的集合。
RandomizedCollection collection = new RandomizedCollection();

// 向集合中插入 1 。返回 true 表示集合不包含 1 。
collection.insert(1);

// 向集合中插入另一个 1 。返回 false 表示集合包含 1 。集合现在包含 [1,1] 。
collection.insert(1);

// 向集合中插入 2 ，返回 true 。集合现在包含 [1,1,2] 。
collection.insert(2);

// getRandom 应当有 2/3 的概率返回 1 ，1/3 的概率返回 2 。
collection.getRandom();

// 从集合中删除 1 ，返回 true 。集合现在包含 [1,2] 。
collection.remove(1);

// getRandom 应有相同概率返回 1 和 2 。
collection.getRandom();

---

276ms

class RandomizedCollection {
    var nums:[Int] = [Int]()
    var m:[Int:Set<Int>] = [Int:Set<Int>]()

    /** Initialize your data structure here. */
    init() {
        
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    func insert(_ val: Int) -> Bool {
        var set:Set<Int> = Set<Int>()
        if m[val] == nil
        {
            set.insert(nums.count)
            m[val] = set
        }
        else
        {
            m[val]!.insert(nums.count)
        }
        nums.append(val)
        return m[val]!.count == 1
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    func remove(_ val: Int) -> Bool {
        var set:Set<Int> = Set<Int>()
        if m[val] != nil
        {
            if m[val]!.isEmpty
            {
                return false
            }
            var idx:Int = m[val]!.first!
            m[val]!.removeFirst()        
        if (nums.count - 1) != idx
        {
            var t:Int = nums.last!
            nums[idx] = t
            m[t]!.remove(nums.count - 1)
            m[t]!.insert(idx)
        }
            nums.removeLast()
            return true
        }
        return false        
    }
    
    /** Get a random element from the collection. */
    func getRandom() -> Int {
        return nums[Int.random(in: 1...nums.count) % nums.count]
    
    }
}

/**
 * Your RandomizedCollection object will be instantiated and called as such:
 * let obj = RandomizedCollection()
 * let ret_1: Bool = obj.insert(val)
 * let ret_2: Bool = obj.remove(val)
 * let ret_3: Int = obj.getRandom()
 */
 

---

280ms

class RandomizedCollection {
    var nums: [Int]
    var locs: [Int: Set<Int>]
    /** Initialize your data structure here. */
    init() {
        nums = []
        locs = [:]
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    func insert(_ val: Int) -> Bool {
        let loc = locs[val]
        let contains = loc != nil
        if !contains { locs[val] = Set<Int>() }
        locs[val]!.insert(nums.count)
        
        nums.append(val)
        return !contains
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    func remove(_ val: Int) -> Bool {
        guard let loc = locs[val],
            let removeIndex = loc.first else { return false }
        
        // remove before adding last num in case they are the same number
        locs[val]!.remove(removeIndex)
        if removeIndex != nums.count - 1 {
            // find last number and replace number we want to remove
            let lastNum = nums.last!
            nums[removeIndex] = lastNum
            
            // remove last num location and add back
            locs[lastNum]!.remove(nums.count-1)
            locs[lastNum]!.insert(removeIndex)
        }
        nums.removeLast()
        return true
    }
    
    /** Get a random element from the collection. */
    func getRandom() -> Int {
        let rand = Int.random(in: 0..<nums.count)
        return nums[rand]
    }
}