题目:
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();

分析:

方法1(bad)

    class RandomizedSet {
        HashSet<Integer> hs;
        /** Initialize your data structure here. */
        public RandomizedSet() {
            hs = new HashSet<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(!hs.contains(val)) {
                hs.add(val);
                return true;
            }
            return false;
        }

        /** Removes a value from the set. Returns true if the set contained the specified element. */
        public boolean remove(int val) {
            if(hs.contains(val)) {
                hs.remove(val);
                return true;
            }
            return false;
        }

        /** Get a random element from the set. */
        public int getRandom() {
            List<Integer> list = new ArrayList<Integer>(hs);
            return list.get(0 + (new Random()).nextInt(list.size()-1 - 0 + 1));
        }
        public static void main(String[] args) {
            RandomizedSet sol = new RandomizedSet();
            sol.insert(1);
            sol.insert(2);
            System.out.println(sol.getRandom());
        }
    }

方法2(better)

import java.util.*;

class RandomizedSet {
    HashMap<Integer, Integer> map1;
    HashMap<Integer, Integer> map2;
    Queue<Integer> keys;
    int newkey;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        map1 = new HashMap<Integer, Integer>();
        map2 = new HashMap<Integer, Integer>();
        keys = new LinkedList<Integer>();
        newkey = 0;
    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        int k;
        if(!map1.containsKey(val)) {
            if(keys.isEmpty()) {
                k = newkey++;
                map1.put(val, k);
            }
            else {
                k = keys.poll();
                map1.put(val, k);
            }
            map2.put(k, val);
            return true;
        }
        return false;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if(map1.containsKey(val)) {
            int k = map1.remove(val);
            keys.add(k);
            map2.remove(k);
            return true;
        }
        return false;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        int random_index = (new Random()).nextInt( newkey);

        while(!map2.containsKey(random_index)) {
            random_index = (new Random()).nextInt( newkey);
        }
        return map2.get(random_index);
    }
}