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LeetCode Top Interview Questions 380. Insert Delete GetRandom O(1) (Java版; Meidum)

题目描述

Design a data structure that supports all following operations in average O(1) time.

insert(val): Inserts an item val to the set if not already present.
remove(val): Removes an item val from the set if present.
getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.
Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

第一次做; new Random().nextInt(6)随机输出[0,6)之间的整数; 覆盖操作的技巧可以降低ArrayList删除元素的开销, 毕竟ArrayList删除最后一个元素不需要大规模调整数组; 待删除的元素, 如果该元素不是最后一个元素则需要执行覆盖操作, 让最后一个元素覆盖当前元素, 然后删掉最后一个元素

class RandomizedSet {
    private List<Integer> list;
    private Map<Integer, Integer> map;
    Random random;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        list = new ArrayList<>();
        map = 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) {
        boolean flag = map.containsKey(val);
        //不考虑重复元素
        if(flag)
            return false;
        //此时list.size()是val的索引
        map.put(val, list.size());
        //加入list
        list.add(val);
        return !flag;
        
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        //先判断val是否存在
        //ArrayList的contains和HashMap的containsKey有什么区别? 平均时间复杂度一样吗?     arraylist和hashmap的remove平均时间复杂度一样吗?
        boolean flag = map.containsKey(val);
        if(!flag)
            return false;
        //
        int index = map.get(val);
        //如果val不是list中的最后一个元素就不用执行覆盖操作了, 直接删除即可
        if(index<list.size()-1){
            //让list中的最后一个元素覆盖index位置的元素, 然后删除list最后一个元素, 这样就不用大规模调整数组了
            int tmp = list.get(list.size()-1);
            list.set(index, tmp);
            //覆盖之后,调整该元素在map中的索引
            map.put(tmp, index);
        }
        //从list中删除
        list.remove(list.size()-1);
        //从map中删除
        //hashmap的remove操作是O(1)吗?
        map.remove(val);
        return true;
    }
    
    /** Get a random element from the set. */
    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();
 */

​​力扣官方题解分析​​

LeetCode Top Interview Questions 380. Insert Delete GetRandom O(1) (Java版; Meidum)_java