# 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 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();
``````

``````class RandomizedSet {
public:
RandomizedSet() {}
bool insert(int val) {
if (m.count(val)) return false;
nums.push_back(val);
m[val] = nums.size() - 1;
return true;
}
bool remove(int val) {
if (!m.count(val)) return false;
int last = nums.back();
m[last] = m[val];
nums[m[val]] = last;
nums.pop_back();
m.erase(val);
return true;
}
int getRandom() {
return nums[rand() % nums.size()];
}
private:
vector<int> nums;
unordered_map<int, int> m;
};
``````

Github 同步地址：

https://github.com/grandyang/leetcode/issues/380

Insert Delete GetRandom O(1) - Duplicates allowed

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

https://leetcode.com/problems/insert-delete-getrandom-o1/discuss/85422/AC-C%2B%2B-Solution.-Unordered_map-%2B-Vector

https://leetcode.com/problems/insert-delete-getrandom-o1/discuss/85401/Java-solution-using-a-HashMap-and-an-ArrayList-along-with-a-follow-up.-(131-ms)

LeetCode All in One 题目讲解汇总(持续更新中…)

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