[LeetCode] 380. Insert Delete GetRandom O(1)

题：https://leetcode.com/problems/insert-delete-getrandom-o1/description/

题目

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

1. insert(val): Inserts an item val to the set if not already present.
2. remove(val): Removes an item val from the set if present.
3. 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();

思路

题目大意

实现一种数据结构，这个数据结构的有三种操作：插入，删除，随机返回。
但要求是所有的操作的时间复杂度都为O(1)。

解题思路

使用 链表可以 实现 插入 O(1)，但是 删除，随机返回 无法满足。
使用 普通数组 可以实现 插入 随机返回 O(1)，但是 删除 无法满足。需要对删除操作 改变数据结构。 设置 List 存储 数据value， map<Integer , Integer > 存储 value -> location 存储数据 在List的位置。当删除value时，将 list最后一个元素换到 value的位置，并在map中替换value。最后删除 map中的value和list的最后一个元素。

code

class RandomizedSet {
    private Map<Integer, Integer> locMap;
    private List<Integer> eleLst;
    Random rand;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        locMap = new HashMap<>();
        eleLst = new ArrayList<>();
        rand = 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(locMap.containsKey(val)) return false;
        eleLst.add(val);
        locMap.put(val,eleLst.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(!locMap.containsKey(val)) return false;        
        
        Integer loc = locMap.get(val);
        Integer tval = eleLst.get(eleLst.size()-1);
        eleLst.set(loc,tval);
        locMap.put(tval,loc);
        locMap.remove(val);
        eleLst.remove(eleLst.size()-1);
        return true;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        
        return eleLst.get(rand.nextInt(eleLst.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();
 */