leetcode 381. Insert Delete GetRandom O(1)

待继续了解....

public class RandomizedCollection {    ArrayList<Integer> nums;HashMap<Integer, Set<Integer>> locs;java.util.Random rand = new java.util.Random();    /** Initialize your data structure here. */    public RandomizedCollection() {        nums = new ArrayList<Integer>();    locs = new HashMap<Integer, Set<Integer>>();    }        /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */    public boolean insert(int val) {        boolean contain = locs.containsKey(val);    if ( ! contain ) locs.put( val, new LinkedHashSet<Integer>() );     locs.get(val).add(nums.size());            nums.add(val);    return ! contain ;    }        /** Removes a value from the collection. Returns true if the collection contained the specified element. */    public boolean remove(int val) {        boolean contain = locs.containsKey(val);    if ( ! contain ) return false;    int loc = locs.get(val).iterator().next();    locs.get(val).remove(loc);    if (loc < nums.size() - 1 ) {       int lastone = nums.get( nums.size()-1 );       nums.set( loc , lastone );       locs.get(lastone).remove( nums.size()-1);       locs.get(lastone).add(loc);    }    nums.remove(nums.size() - 1);       if (locs.get(val).isEmpty()) locs.remove(val);    return true;    }        /** Get a random element from the collection. */    public int getRandom() {        return nums.get( rand.nextInt(nums.size()) );    }}