380. Insert Delete GetRandom O(1)

Design a data structure that supports all following operations inaverage 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();

Solution 1: HashMap + ArrayList O(1); O(n)

代码包括了381的代码，方便对比。

    /**
     * HashMap + ArrayList O(1); O(n)
     * Use ArrayList to store all values
     * Use HashMap to store values and their indexes in ArrayList
     * Every insert : index + 1
     * Every remove : Put the last value in the removed location and remove the last one
     */
    Map<Integer, Integer> map;
    //    HashMap<Integer, Set<Integer>> map;
    List<Integer> nums;
    Random rand;

    //    /** Initialize your data structure here. */
    public InsertDeleteGetRandom() {
        map = new HashMap<>();
//        map = new HashMap<Integer, Set<Integer>>();
        nums = new ArrayList<>();
        rand = new Random();
    }

    //    /** 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;
        }
//        if (!map.containsKey(val)) {
//            map.put(val, new HashSet<Integer>());
//        }

        map.put(val, nums.size());
//        map.get(val).add(nums.size());
        nums.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);
        //no need to add this code: if (index < nums.size() - 1)
        //not the last one than swap the last one with this val
        int last = nums.get(nums.size() - 1);
        map.put(last, index);
        nums.set(index, last);
//        int index = map.get(val).iterator().next(); //get index of val in ArrayList
//        map.get(val).remove(index); //remove index of val in HashSet
//        if (index < nums.size() - 1) { //this code is necessary here, otherwise will lead to out of bound
////        int last = nums.get(nums.size() - 1 ); //get the last val
////        nums.set(index , last); //replace val by last val
////        map.get(last).remove(nums.size() - 1); // remove index of last val
////        map.get(last).add(index); //add new index of last val
//        }


        map.remove(val);
//        if (map.get(val).isEmpty()) map.remove(val); //if val's HashSet is empty, remove key of val
        nums.remove(nums.size() - 1);
        return true;
    }

    //    Get a random element from the set.
    public int getRandom() {
        return nums.get(rand.nextInt(nums.size()));
    }

Follow Up:

1.allow duplicates 相关面试题(L): http://www.1point3acres.com/bbs/thread-156459-1-1.html

Solution 1: HashMap + HashSet + ArrayList O(1); O(n)

The idea is to change the value of hashMap from Integer to HashSet to remember all the locations of a duplicated number.

    Map<Integer, Set<Integer>> map;
    List<Integer> nums;
    Random rand;

    ///** Initialize your data structure here. */
    public InsertDeleteGetRandomII() {
        map = new HashMap<>();
        nums = new ArrayList<>();
        rand = new Random();
    }

    ///** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        if (!map.containsKey(val)) {
            map.put(val, new HashSet<>());
            map.get(val).add(nums.size());
            nums.add(val);
            return true;
        }
        map.get(val).add(nums.size());
        nums.add(val);
        return false;
    }

    ///** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        if (!map.containsKey(val)) {
            return false;
        }

        int index = map.get(val).iterator().next();
        map.get(val).remove(index);
        if (index < nums.size() - 1) {
            int last = nums.get(nums.size() - 1);
            nums.set(index, last);
            map.get(last).remove(nums.size() - 1);
            map.get(last).add(index);
        }

        if (map.get(val).isEmpty()) {
            map.remove(val);
        }
        nums.remove(nums.size() - 1);
        return true;
    }

    ///** Get a random element from the collection. */
    public int getRandom() {
        return nums.get(rand.nextInt(nums.size()));
    }