LeetCode Daily Question 10

3 月 15 日 每日一题

题目：380. Insert Delete GetRandom O(1)

Implement the RandomizedSet class:

RandomizedSet() Initializes the RandomizedSet object.
bool insert(int val) Inserts an item val into the set if not present. Returns true if the item was not present, false otherwise.
bool remove(int val) Removes an item val from the set if present. Returns true if the item was present, false otherwise.
int getRandom() Returns a random element from the current set of elements (it’s guaranteed that at least one element exists when this method is called). Each element must have the same probability of being returned.
You must implement the functions of the class such that each function works in average O(1) time complexity.

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Example 1:

Input
["RandomizedSet", "insert", "remove", "insert", "getRandom", "remove", "insert", "getRandom"]
[[], [1], [2], [2], [], [1], [2], []]
Output
[null, true, false, true, 2, true, false, 2]

Explanation
RandomizedSet randomizedSet = new RandomizedSet();
randomizedSet.insert(1); // Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomizedSet.remove(2); // Returns false as 2 does not exist in the set.
randomizedSet.insert(2); // Inserts 2 to the set, returns true. Set now contains [1,2].
randomizedSet.getRandom(); // getRandom() should return either 1 or 2 randomly.
randomizedSet.remove(1); // Removes 1 from the set, returns true. Set now contains [2].
randomizedSet.insert(2); // 2 was already in the set, so return false.
randomizedSet.getRandom(); // Since 2 is the only number in the set, getRandom() will always return 2.

思路

本题使用 hashmap 记录每一个 val 在 vector 的 index，以达到 O(1)的 search。 O(1) remove 通过将 val 位置和 vector 末尾值的位置调换再 pop 实现，需要注意的是 swap 之前需要先更新 hashmap 内 vector 末尾值的 index。getRomdom 通过 rand()%nums.size()实现。

代码

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class RandomizedSet {
public:
    RandomizedSet() {}

    bool insert(int val) {
        if(indexs.find(val) != indexs.end())
            return false;
        nums.push_back(val);
        indexs[val] = int(nums.size() - 1);
        return true;
    }

    bool remove(int val) {
        if(indexs.find(val) == indexs.end())
            return false;
        indexs[nums[nums.size() - 1]] = indexs[val];
        swap(nums[indexs[val]], nums[nums.size() - 1]);
        nums.pop_back();
        indexs.erase(val);
        return true;
    }

    int getRandom() {
        return nums[rand() % nums.size()];
    }
private:
    unordered_map<int,int> indexs;
    vector<int> nums;
};