[LeetCode] 380. Insert Delete GetRandom O(1) 插入删除获得随机数O(1)时间

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

 

1. insert(val): Inserts an item val to the set if not already present.
2. remove(val): Removes an item val from the set if present.
3. getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.

 

Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();

设计一个数据结构在O(1)时间内完成：insert(val)，remove(val)，getRandom()

由于是O(1)时间，要用哈希表 + 数组(HashMap + Array)，数组用来保存数字，哈希表用来建立每个数字和其在数组中的位置之间的映射。

插入操作：先判断是否在HashMap里，如果存在则不需要插入，直接返回false。不存在的话，把此数插入到数组的末尾，然后在HashMap中建立数字和其位置的映射。

删除操作：先判断是否在HashMap里，如果没有，直接返回false。由于HashMap的删除是O(1)时间，而数组并不是。为了保证了O(1)时间内的删除，把要删除的数字和数组的最后一个数调换个位置，然后修改对应的HashMap中的值，删除数组的最后一个元素即可。

返回随机数：随机生成一个数组位置，返回该位置上的数字。

Java:

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public class RandomizedSet {     Map<Integer, Integer> map;//<val, index in arrlist>     List<Integer> list;//keep a record of value for get random     int size;//map and list are the same size     Random rand;     /** Initialize your data structure here. */     public RandomizedSet() {         this.map = new HashMap<>();         this.list = new ArrayList<>();         this.size = 0;         this.rand = new Random();     }           /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */     public boolean insert(int val) {         if(map.containsKey(val)) return false;         map.put(val, size);         list.add(val);         size++;         return true;     }           /** Removes a value from the set. Returns true if the set contained the specified element. */     public boolean remove(int val) {         if(map.containsKey(val)){             int last = list.get(size - 1);             list.set(map.get(val), last);//use last added ele to fill the element to be removed             map.put(last, map.get(val));//update last added ele's index in list             map.remove(val);             list.remove(size - 1);//remove at end is constant             size--;             return true;         }         return false;     }           /** Get a random element from the set. */     public int getRandom() {         return list.get(rand.nextInt(size));     } }

Python:

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import random class RandomizedSet(object):       def __init__(self):         """         Initialize your data structure here.         """         self.dataMap = {}         self.dataList = []       def insert(self, val):         """         Inserts a value to the set. Returns true if the set did not already contain the specified element.         :type val: int         :rtype: bool         """         if val in self.dataMap:             return False         self.dataMap[val] = len(self.dataList)         self.dataList.append(val)         return True       def remove(self, val):         """         Removes a value from the set. Returns true if the set contained the specified element.         :type val: int         :rtype: bool         """         if val not in self.dataMap:             return False         idx = self.dataMap[val]         tail = self.dataList.pop()         if idx < len(self.dataList):             self.dataList[idx] = tail             self.dataMap[tail] = idx         del self.dataMap[val]         return True       def getRandom(self):         """         Get a random element from the set.         :rtype: int         """         return random.choice(self.dataList)

C++:

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class RandomizedSet { public:     /** Initialize your data structure here. */     RandomizedSet() {}           /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */     bool insert(int val) {         if (m.count(val)) return false;         nums.push_back(val);         m[val] = nums.size() - 1;         return true;     }           /** Removes a value from the set. Returns true if the set contained the specified element. */     bool remove(int val) {         if (!m.count(val)) return false;         int last = nums.back();         m[last] = m[val];         nums[m[val]] = last;         nums.pop_back();         m.erase(val);         return true;     }           /** Get a random element from the set. */     int getRandom() {         return nums[rand() % nums.size()];     } private:     vector<int> nums;     unordered_map<int, int> m; };

 

类似题目：

[LeetCode] 381. Insert Delete GetRandom O(1) - Duplicates allowed 插入删除和获得随机数O(1)时间 - 允许重复　　

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