381. Insert Delete GetRandom O(1) - Duplicates allowed-优快云博客

本文介绍了一种特殊的数据结构，该结构支持插入、移除和随机返回集合中元素的操作，并且所有操作的平均时间复杂度为O(1)。此结构允许元素重复，随机返回的概率与相同值的数量成线性关系。

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

Note: Duplicate elements are allowed.

insert(val): Inserts an item val to the collection.
remove(val): Removes an item val from the collection if present.
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();

这里的结构允许出现重复的数字,且在随机选取某个数时,概率相等.

仍然用hash表，但记录val在数组中的位置用set。

class RandomizedCollection {
   map<int,set<int>> locs;//val to loc
    vector<int> nums;
public:
    /** Initialize your data structure here. */
    RandomizedCollection() {
        
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    bool insert(int val) {
        nums.push_back(val);
        locs[val].insert(nums.size()-1);
        if(locs[val].size()==1)
        return 1;
        else return 0;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    bool remove(int val) {
       if(locs.find(val)==locs.end()) return 0;
       int idx=*locs[val].begin();
       int last=nums[nums.size()-1];
       nums[idx]=last;
       
       locs[val].erase(idx);//这里必须先删除一个才可以插入，如果last和val相等，先插入，会没有效果。这里调试了好久才找到错误
       locs[last].insert(idx);
       locs[last].erase(nums.size()-1);
       nums.pop_back();
      
       
      if(locs[val].size()==0) locs.erase(val);
       return 1;
      
    }
    
    /** Get a random element from the collection. */
    int getRandom() {
      return nums[rand()%nums.size()];  
    }
};