Design a data structure that supports all following operations in average O(1) time.
Note: Duplicate elements are allowed.
insert(val)
: Inserts an item val to the collection.remove(val)
: Removes an item val from the collection if present.getRandom
: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains.
Example:
// Init an empty collection. RandomizedCollection collection = new RandomizedCollection(); // Inserts 1 to the collection. Returns true as the collection did not contain 1. collection.insert(1); // Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1]. collection.insert(1); // Inserts 2 to the collection, returns true. Collection now contains [1,1,2]. collection.insert(2); // getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3. collection.getRandom(); // Removes 1 from the collection, returns true. Collection now contains [1,2]. collection.remove(1); // getRandom should return 1 and 2 both equally likely. collection.getRandom();
算法:试了一下用双Map无法实现频率正比概率random。当然如果random的概率不和频率成正比而是等概率的话反而还真的必须得用双map。
最后使用Map + List。和前面的区别在于Map<Integer, Set<Integer>>,维护的映射从存储单个int下标信息变成了用一个Set存储下标信息。 删的时候比较麻烦。细节不写好容易报错。
1.删待删数对应的一个下标(如果set空了记得清空整个key)。
2.如果之前删的下标不是最后一个,那你还要把最后一个数提到下标那里(更新该val的下标信息要一加一删,切记),然后在list里也更新一下。
3.在List里把最后一个数删掉,size更新。返回
实现:
class RandomizedCollection { private List<Integer> list; private Map<Integer, Set<Integer>> vi; private int size; /** Initialize your data structure here. */ public RandomizedCollection() { list = new ArrayList<Integer>(); vi = new HashMap<Integer, Set<Integer>>(); size = 0; } /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */ public boolean insert(int val) { boolean contains = vi.containsKey(val); if (!contains) { vi.put(val, new LinkedHashSet<>()); } vi.get(val).add(size); list.add(val); size++; // 注意是返回!contains!,相反的 return !contains; } /** Removes a value from the collection. Returns true if the collection contained the specified element. */ public boolean remove(int val) { if (!vi.containsKey(val)) { return false; } // 删原来那个的下标 int index = vi.get(val).iterator().next(); vi.get(val).remove(index); if (vi.get(val).size() == 0) { vi.remove(val); } // 如果需要换的话把换到前面后的下标信息更新 if (index < size - 1) { int lastVal = list.get(size - 1); //这句不要漏了啊啊啊!更新最后一个的坐标要一加一删 vi.get(lastVal).remove(size - 1); vi.get(lastVal).add(index); list.set(index, lastVal); } list.remove(size - 1); size--; return true; } /** Get a random element from the collection. */ public int getRandom() { return list.get(new Random().nextInt(size)); } } /** * Your RandomizedCollection object will be instantiated and called as such: * RandomizedCollection obj = new RandomizedCollection(); * boolean param_1 = obj.insert(val); * boolean param_2 = obj.remove(val); * int param_3 = obj.getRandom(); */