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### 题目描述

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.

### 例子

#### 例子 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.`

### Constraints

• `-2^31 <= val <= 2^31 - 1`
• At most `2 * 10^5` calls will be made to `insert`, `remove`, and `getRandom`.
• There will be at least one element in the data structure when `getRandom` is called.

### 解题思路

• 随机读取元素，所以存储方式肯定要是数组，这里用 vector
• 常数时间查找元素，所以需要用哈希表存储下标
• 在去除的时候需要在常数时间内去除，并且还要保持数组紧凑（因为要保证 getRamdon 可以常数时间内获得）因此，在去除的时候可以先把元素交换到数组末尾在进行常数时间的pop out

• 时间复杂度: `O(1)`
• 空间复杂度: `O(n)`

GitHub 代码同步地址： 380.InsertDeleteGetRandomO1.cpp

GitHub: Leetcode-C++-Solution