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();

一刷
题解：构造一个array和一个map, map中的key为val, value为val在array中的位置。
如果取出来的是array中的最后一个元素，直接从map和array中移去。如果不是，则和最后一个元素交换位置，然后移去。
每次get时，rand取出[0-array.size()-1]中的一个随机数，表示array中的index。将对应元素从array中移去。

public class RandomizedSet {
    ArrayList<Integer> nums;
    HashMap<Integer, Integer> locs;
    java.util.Random rand = new java.util.Random();
    /** Initialize your data structure here. */
    public RandomizedSet() {
        nums = new ArrayList<Integer>();
        locs = 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) {
        boolean contain = locs.containsKey(val);
        if ( contain ) return false;
        locs.put( val, nums.size());//indicate the place of the val in the array
        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) {
        boolean contain = locs.containsKey(val);
        if ( ! contain ) return false;
        int loc = locs.get(val);
        if (loc < nums.size() - 1 ) { // not the last one than swap the last one with this val
            int lastone = nums.get(nums.size() - 1 );
            nums.set( loc , lastone );
            locs.put(lastone, loc);
        }
        locs.remove(val);
        nums.remove(nums.size() - 1);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return nums.get( rand.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();
 */