Link: https://leetcode.cn/problems/insert-delete-getrandom-o1/

Question

difficulty: mid
adj diff: 4

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.

 

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.

 

Constraints:

    -231 <= val <= 231 - 1
    At most 2 * 105 calls will be made to insert, remove, and getRandom.
    There will be at least one element in the data structure when getRandom is called.

这题有一点难。

思路：一个array + 一个 map。

其中，map存贮的是 { value -> index } 这个 pair。

remove 一个数字的时候，把 array 尾部的元素拿到 remove的那个地方。

Code

直接看代码吧。

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class RandomizedSet {

    int[] array = new int[200000];
    // map is a k-v pair of { value -> index }
    Map<Integer, Integer> map = new HashMap<Integer, Integer>();

    int size;
    Random random = new Random();

    public RandomizedSet() {}
    
    public boolean insert(int val) {
        if (map.containsKey(val)) {
            return false;
        }
        map.put(val, size);
        array[size++] = val;
        return true;
    }
    
    public boolean remove(int val) {
        if (!map.containsKey(val)) {
            return false;
        }
        int removeIndex = map.remove(val);
        array[removeIndex] = array[--size];
        if (removeIndex != size) {
            // this is a special case, be cautious
            map.put(array[removeIndex], removeIndex);
        }
        return true;
    }
    
    public int getRandom() {
        return array[random.nextInt(size)];
    }
}