算法

(设计+数组+哈希) $O(1)$

工具类的使用。

Java 代码

class RandomizedSet {

    // array: vals
    List<Integer> nums;
    // map: (vals, idx)
    Map<Integer, Integer> map;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        nums = new ArrayList();
        map = new HashMap();
    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (map.containsKey(val)) return false;
        nums.add(val);
        map.put(val, nums.size() - 1);
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (!map.containsKey(val)) return false;
        int idx = map.get(val);
        int n = nums.size();
        int temp = nums.get(n - 1);
        nums.set(idx, temp);
        nums.remove(n - 1);
        map.put(temp, idx);
        map.remove(val);
        return true;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        Random rand = new Random();
        return nums.get(rand.nextInt(nums.size()));
    }
}