leetcode-380- 常数时间插入、删除和获取随机元素

class RandomizedSet {
    // 存储元素
    ArrayList<Integer> nums;
    // 存储元素和下标的映射关系
    HashMap<Integer, Integer> valToIndex;
    // 随机数生成器
    Random random;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        nums = new ArrayList<>();
        valToIndex = new HashMap<>();
        random = new Random();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (valToIndex.containsKey(val)) {
            return false;
        }
        valToIndex.put(val, nums.size());
        nums.add(val);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (!valToIndex.containsKey(val)) {
            return false;
        }
        int index = valToIndex.get(val);
        // 若删除的元素不是最后一个元素，与最后一个元素进行交换
        if ( index != nums.size() - 1) {
            int last = nums.get(nums.size() - 1);
            nums.set(index, last);
            valToIndex.put(last, index);
        }
        // 删除元素对应的映射关系
        valToIndex.remove(val);
        // 删除元素
        nums.remove(nums.size() - 1);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
    	// 随机获取 nums 中元素的下标对应的值
        return nums.get(random.nextInt(nums.size()));
    }
}

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */