LeetCode 380. Insert Delete GetRandom O(1)

380. Insert Delete GetRandom O(1)

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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();

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【题目分析】

设计一种数据结构，使得插入，删除和随机获取一个值的时间复杂度为O(1).

【思路】

1. 随机读取一个数的话，使用数组的时间复杂度是最小的，因为通过下标可以直接定位。考虑到元素会不断插入，我们可以选择ArrayList来存储我们的元素。

2. 要删除一个元素，在数组中需要遍历才能找到这个元素。而在HashMap中，可以快速定位一个元素。因此我们可以用HashMap来存储元素和它在ArrayList中对应的下标。

通过以上的分析，我们知道了数据结构该如何设计。

【java代码】

public class RandomizedSet {
    Map<Integer, Integer> map;
    List<Integer> list;
    Random random;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap<>();
        list = new ArrayList<>();
        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(map.containsKey(val)) return false;
        map.put(val, list.size());
        list.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(!map.containsKey(val)) return false;
        int loc = map.get(val);
        if(loc < list.size()-1) {
            int last = list.get(list.size()-1);
            list.set(loc, last);
            map.put(last, loc);
        }
        map.remove(val);
        list.remove(list.size()-1);
        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();
 */