380. Insert Delete GetRandom O(1) - jiejackyzhang/leetcode-note GitHub Wiki

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 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();

这道题要求insert，delete，getRandom三个操作的时间都是O(1)。

insert，delete可以通过hash table实现，如果要getRandom也需要O(1)，我们需要使用ArrayList来存储所有元素，通过random的下标可以实现O(1)的getRandom，而且概率相等。

因此我们要用一个hash map来记录元素的下标，查询map的key可以知道元素是否存在。

每次insert一个新元素，都加在list的末尾；每次remove一个元素，如果不在末尾，则将末尾元素与其交换，从而删除list的末尾。 这样insert，delete操作也都是O(1)。

public class RandomizedSet {
    
    private List<Integer> nums;
    private Map<Integer, Integer> locs;
    private Random random;

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