LeetCode 380. Insert Delete GetRandom O(1)

原题链接在这里：https://leetcode.com/problems/insert-delete-getrandom-o1/?tab=Description

题目：

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();

题解：

用list保存insert进来的数字. 利用HashMap<Integer, Integer> hm保存insert进来数字和对应的index.

insert, remove时对应更改list, hm.

getRandom在list上用random index get一个数.

Time Complexity: inset, O(1). remove, O(1). getRandom, O(1).

Space: O(n).

AC Java:

public class RandomizedSet {
    List<Integer> ls;
    HashMap<Integer, Integer> hm;
    Random rand;
    
    /** Initialize your data structure here. */
    public RandomizedSet() {
        ls = new ArrayList<Integer>();
        hm = new HashMap<Integer, Integer>();
        rand = 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(hm.containsKey(val)){
            return false;
        }
        ls.add(val);
        hm.put(val, ls.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(!hm.containsKey(val)){
            return false;
        }
        
        int ind = hm.get(val);
        //swap当前数字和ls的最后一个数字，方便在list上remove当前数字时不影响其他数字index
        if(ind < ls.size()-1){
            int lastElement = ls.get(ls.size()-1);
            ls.set(ind, lastElement);
            hm.put(lastElement, ind);
        }
        
        hm.remove(val);
        ls.remove(ls.size()-1);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return ls.get(rand.nextInt(ls.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();
 */

 跟上Insert Delete GetRandom O(1) - Duplicates allowed.