LC.P380[O(1)时间插入、删除和获取随机元素]

方法一：Map+List

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class RandomizedSet {

    Map<Integer, Integer> map;
    List<Integer> list;
    Random random;

    public RandomizedSet() {
        map = new HashMap<>();
        list = new ArrayList<>();
        random = new Random();
    }
    
    public boolean insert(int val) {
        if (map.containsKey(val)) return false;
        map.put(val, list.size());
        list.add(val);
        return true;
    }
    
    public boolean remove(int val) {
        if (!map.containsKey(val)) return false;
        int size = list.size();
        Integer lastVal = list.get(size - 1);
        int index = map.get(val);
        list.set(index, lastVal);
        map.put(lastVal, index);
        list.remove(size - 1);
        map.remove(val);
        return true;
    }
    
    public int getRandom() {
        return list.get(random.nextInt(list.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();
 */
  • 时间复杂度：$O(1)$
  • 空间复杂度：$O(n)$