[LeetCode] Insert Delete GetRandom O(1)

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 probabilityof 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();

代码如下：

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