Leetcode 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();

Analysis:

We can use a hashMap + array here. 

Insert - insert the val to the tail of the array, and update the hash map.

Remove - swap the val to be deleted with the tail of the list node. Update the index in the hash map, and remove tail of the array

GetRandom - Get a random index in the array and return its value.

Code (Java):

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class RandomizedSet {     private List<Integer> list;     private Map<Integer, Integer> map;           /** Initialize your data structure here. */     public RandomizedSet() {         list = new ArrayList<>();         map = 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 (map.containsKey(val)) {             return false;         }                   int index = list.size();         list.add(val);         map.put(val, index);                   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 indexRemove = map.get(val);         int tail = list.get(list.size() - 1);                   swap(indexRemove, list.size() - 1);         map.put(tail, indexRemove);         list.remove(list.size() - 1);         map.remove(val);                   return true;     }           /** Get a random element from the set. */     public int getRandom() {                   if (list.isEmpty()) {             return 0;         }                   Random rand = new Random();         int index = rand.nextInt(list.size());                   return list.get(index);     }           private void swap(int i, int j) {         int temp = list.get(i);         list.set(i, list.get(j));         list.set(j, temp);     } }   /**  * 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();  */