leetcode380. Insert Delete GetRandom O(1)

380. Insert Delete GetRandom O(1)

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();

解法

使用一个list和一个map，list中存放元素，map中存放key：list中元素的值和value：在list中的位置。方便list进行O(1)时间复杂度的删除。

public class RandomizedSet {    List<Integer> nums;    Map<Integer, Integer> loc;    Random random;    /** Initialize your data structure here. */    public RandomizedSet() {        nums = new ArrayList<Integer>();        loc = new HashMap<Integer, Integer>();        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 (loc.containsKey(val)) {            return false;        }        nums.add(val);        loc.put(val, nums.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 (!loc.containsKey(val)) {            return false;        }        int position = loc.get(val);        if (position < nums.size() - 1) {            int lastnode = nums.get(nums.size() - 1);            nums.set(position, lastnode);            loc.put(lastnode, position);        }        nums.remove(nums.size() - 1);        loc.remove(val);        return true;    }    /** Get a random element from the set. */    public int getRandom() {        return nums.get(random.nextInt(nums.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(); */