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()) ); } }
