June Leetcoding Challenge(6.12)

Leetcode六月挑战赛（6.12）

Leetcode380 Insert Delete GetRandom O(1)

这是之前做过的一道题，我们再来详细分析一下。

题目描述

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：

Hashmap存放加入的val和在list的位置，list中存储每一个val。则每一个val对应的index为map.get(val)。如果index为null或者list.get(index)为null则代表list中没有该元素，若调用insert加入并返回true, 若调用remove则返回false。定义一个random对象，随机获取index，若list.get(index)为空，则重新随机获取。

代码

class RandomizedSet {
    
    HashMap<Integer,Integer> map;
    List<Integer> list;
    Random rd = new Random();
    /** Initialize your data structure here. */
    public RandomizedSet() {
        map =  new HashMap<>();
        list =  new ArrayList<>();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        Integer index = map.get(val);
        if(index != null && list.get(index)!=null )
            return false;
        else{
            list.add(val);
            map.put(val,list.size()-1);
            return true;
        }
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        Integer index = map.get(val);
        if(index!=null&& list.get(index)!=null){
            list.set(index,null);
            return true;
        }else
            return false;
        
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        Integer index = rd.nextInt(list.size());
        while(list.get(index)==null){
            index = rd.nextInt(list.size());
        }
        return list.get(index);
    }
}