380. 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 probability of being returned.

设计一个数据结构，使得插入、删除，随机获取一个元素的时间复杂度都是O(1) 

class RandomizedSet {

    ArrayList<Integer> nums; //存放具体的值
    HashMap<Integer, Integer> locs; //标记每个值在nums中的位置
    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());
        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 );//将最后一个位置的元素拷贝到本位置，达到覆盖被删除元素的目的
            locs.put(lastone, loc); //因为map中最后一个位置的元素被移走了，所以要同步修改mamp中的位置索引
        }
        locs.remove(val); //删除map中要被删除元素
        nums.remove(nums.size() - 1);//删除list中最后一个元素
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return nums.get( rand.nextInt(nums.size()) );
    }
}