380.Insert Delete GetRandom O(1)

Tags: Medium Array Hash Table Design

Links: https://leetcode.com/problems/insert-delete-getrandom-o1/

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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();

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用数组存储元素，用哈希表存储元素和下标之间的对应关系，主要是为了降低查找的时间复杂度。

插入就是在数组末尾插入元素，然后在哈希表中添加记录，当然了还是需要先检查元素是否应经存在于数组中。

删除操作就有点技巧了，假设元素存在于数组，那么就把需要删除的元素和数组末尾的元素交换，同时更新哈希表中的信息，然后数组和哈希表执行删除操作，常数时间复杂度。

class RandomizedSet {
    unordered_map<int, int> um;
    vector<int> seq;
public:
    /** Initialize your data structure here. */
    RandomizedSet() {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);
    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    bool insert(int val) {
        if (um.find(val) != um.end()) return false;
        seq.push_back(val);
        um[val] = (int)seq.size() - 1;
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if (um.find(val) == um.end()) return false;
        int n = seq.size();
        int pos = um[val];
        std::swap(seq[n - 1], seq[pos]); //这时候需要删除的元素在数组末尾
        um[seq[pos]] = pos;
        um.erase(seq[n - 1]);
        seq.pop_back();
        return true;
    }

    /** Get a random element from the set. */
    int getRandom() {
        int n = seq.size();
        return seq[rand() % n];
    }
};

/**
 * 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();
 */