[LeetCode 380] Insert Delete GetRandom O(1) (Medium)

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

• insert(val): Inserts an item val to the set if not already present.
• remove(val): Removes an item val from the set if present.
  -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();

Solution: ArrayList + HashMap

1. ArrayList: 用于存放存进来的number
2. HashMap： 用于存储每个数字在ArrayList中的Index （HashMap才能达到所有操作为O(1）)
3. Insert: 向ArrayList尾部加入元素，然后写入HashMap
4. Remove：
   • 从HashMap中找到remove元素在ArrayList中的Index，然后将其与尾部的元素交换。
   • 删除尾部元素，
   • 同时更新HashMap: 删除remove掉的Key Value Pair，同时更新尾部元素的Index （其index变成了被remove掉元素的index）

class RandomizedSet {
    
    List nums;
    Map numberVsIndex; 

    /** Initialize your data structure here. */
    public RandomizedSet() {
        this.nums = new ArrayList ();
        this.numberVsIndex = new HashMap ();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (this.numberVsIndex.getOrDefault (val, -1) != -1) {
            return false;
        }
        
        this.nums.add (val);
        this.numberVsIndex.put (val, nums.size () - 1);
        //System.out.println (nums.toString ());
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (this.numberVsIndex.getOrDefault (val, -1) == -1) {
            return false;
        }
        
        int removeIndex = this.numberVsIndex.get (val);
        int lastItem = this.nums.get (this.nums.size () - 1);
        this.nums.set (removeIndex, lastItem);
        this.numberVsIndex.put (lastItem, removeIndex);
        
        this.nums.remove (this.nums.size () - 1);
        this.numberVsIndex.remove (val);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        int result = 0;
        Random rand = new Random ();
        
        return this.nums.get (rand.nextInt (this.nums.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();
 */