380. Insert Delete GetRandom O(1)

• Difficulty: Medium

• Topics: Array, Hash Table, Design

• Similar Questions:

  • Insert Delete GetRandom O(1) - Duplicates allowed

Problem:

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(); </pre> </p>

Solutions:

class RandomizedSet {
public:
    /** Initialize your data structure here. */
    RandomizedSet() {
        tail = 0;
    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    bool insert(int val) {
        if (numToIndex.count(val) > 0)  return false;
        if (tail == nums.size()) {
            nums.push_back(val);
            numToIndex[val] = tail;
            ++tail;
        } else {
            nums[tail] = val;
            numToIndex[val] = tail;
            ++tail;
        }

        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if (numToIndex.count(val) == 0) return false;
        int lastValue = nums[tail - 1];
        int index = numToIndex[val];
        swap(nums[index], nums[tail - 1]);
        numToIndex[lastValue] = index;
        --tail;
        numToIndex.erase(val);
        return true;
    }

    /** Get a random element from the set. */
    int getRandom() {
        int rIndex = rand() % tail;
        return nums[rIndex];
    }

private:
    vector<int> nums;
    int tail;
    unordered_map<int, int> numToIndex;

};

/**
 * 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();
 */