Insert Delete GetRandom O(1)


#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();


#2

The below code uses a hash-map and a slice. HashMap provides us the O(1) deletion/insertion time, while keeping the keys in a slice/array list provides us a way to randomized O(1). The key to this algorithm is we store the location of the value we are going to insert as a value to the key in the map.

import "math/rand"

type RandomizedSet struct {
    rs map[int]int
    keys []int
}


/** Initialize your data structure here. */
func Constructor() RandomizedSet {
    return RandomizedSet{rs :  make(map[int]int), keys: make([]int, 0)}
}


/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
func (this *RandomizedSet) Insert(val int) bool {
    if _, ok := this.rs[val]; !ok {
        this.rs[val] = len(this.keys)
        this.keys = append(this.keys, val)
        return true
    }
    return false
}


/** Removes a value from the set. Returns true if the set contained the specified element. */
func (this *RandomizedSet) Remove(val int) bool {
    if _, ok := this.rs[val];!ok {
        return false
    }
    loc := this.rs[val]
    if loc < len(this.keys)-1{
        // set the last valuen to current loc, so we could delete last val
        lastOne := this.keys[len(this.keys)-1] 
        this.keys[loc] = lastOne 
        this.rs[lastOne] = loc
    }
    this.keys = this.keys[0:len(this.keys)-1]
    delete(this.rs, val)
    return true
}


/** Get a random element from the set. */
func (this *RandomizedSet) GetRandom() int {
    return this.keys[rand.Intn(len(this.rs))]
}


/**
 * Your RandomizedSet object will be instantiated and called as such:
 * obj := Constructor();
 * param_1 := obj.Insert(val);
 * param_2 := obj.Remove(val);
 * param_3 := obj.GetRandom();
 */

#3

Uses similar logic to use hash map and a list to store keys.

class RandomizedSet {

    Map<Integer, Integer> map;
    List<Integer> list;
    java.util.Random rand = new java.util.Random();
    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap<Integer, Integer>();
        list = new ArrayList<Integer>();
    }
    
    /** 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, list.size());
        list.add(val);
        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)) return false;
         int index = map.get(val);
         if(index < list.size()-1){
            int last = list.get(list.size()-1);
            list.set(index, last);
            map.put(last, index);
         }
        list.remove(list.size()-1);
        map.remove(val);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
       return list.get(rand.nextInt(list.size()) );
    }
}

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