380. Insert Delete GetRandom O(1)

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 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom(); 

---

1. 对于insert和remove，需要在O(1)的时间内查看该value是否存在，并insert/remove该value，因此考虑hash table。
2. 对于getRandom，考虑用array来实现，因为：1）access每个元素的probablity相同；2）可以在O(1)时间内access任意元素。

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(map.find(val)==map.end()){ //val not presented in the set
            map[val]=data.size();
            data.push_back(val);
            return true;
        }
        
        return false;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if(map.find(val)!=map.end()){ // val presented in the set
            int index = map[val];
            map.erase(val);
            
            //swap the index of the removed value with the last element's index
            int lastIndex = data.size()-1;
27             if(index!=lastIndex){
28                 map[data[lastIndex]] = index;
29                 data[index]=data[lastIndex];
30             }
            data.pop_back();
            return true;
        }
        
        return false;
    }
    
    /** Get a random element from the set. */
    int getRandom() {
        return data[rand()%data.size()];
    }
private:
    vector<int> data;           //data[index] = val
    unordered_map<int,int> map; //map[val] = index
};

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * bool param_1 = obj.insert(val);
 * bool param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */

 

Remove分兩步： 

1. 把val从hash table里除去，hash  table里的key是val，value是array里相应的index。

2.把val从array里除去，其中要除去的index是从hash table里查出的。具体做法是：将array[index]和array的最后的index互换，然后将array的size减一。因为array[index]和array[size-1]互换，hash table里的array[index] val也需要更新。

注意：当index是size-1时，不需要做：array[index]和array[size-1]互换，hash table里的array[index] val更新

 

方法二：

先将hash table更新，然后把hash table和array里的元素remove掉

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(map.find(val)!=map.end()){ //val already exists
            return false;
        }
        array.push_back(val);
        map[val]=array.size()-1;
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if(map.find(val)==map.end()){ //val not exist
            return false;
        }
        
24         int index=map[val];
25         map[array[array.size()-1]]=index;
26         array[index]=array[array.size()-1];
27         array.pop_back();
28         map.erase(val);
        
        return true;
    }
    
    /** Get a random element from the set. */
    int getRandom() {
        if(array.size()>0){
            int index=rand()%array.size();
        
            return array[index];
        }
        return 0;
    }
private:
    unordered_map<int,int> map;
    vector<int> array;
};

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * bool param_1 = obj.insert(val);
 * bool param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */