Insert Delete GetRandom O(1) I & II

Design a data structure that supports all following operations in 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.

分析：

因为要求所有操作O(1),插入可以满足这个条件，如果用数组来存储值，getRandom可以满足O(1).但是要求不能重复和delete的时候也满足这个条件，只能使用一个map. key是值，value是位置。

 public class RandomizedSet {
     ArrayList<Integer> nums;//值
     // value to position
     HashMap<Integer, Integer> valueToPositionMap;
     Random rand;

     public RandomizedSet() {
         nums = new ArrayList<>();
         valueToPositionMap = new HashMap<>();
         rand = new Random(System.currentTimeMillis());
     }

     /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
     public boolean insert(int val) {
         boolean isContained = valueToPositionMap.containsKey(val);
         if ( isContained ) return false;
         valueToPositionMap.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 isContained = valueToPositionMap.containsKey(val);
         if ( ! isContained ) return false;
         int position = valueToPositionMap.get(val);
         if (position != nums.size() -  ) {
             // put the true last one to 'position' in nums
             int lastOne = nums.get(nums.size() -  );
             nums.set( position , lastOne );
             valueToPositionMap.put(lastOne, position);
         }
         valueToPositionMap.remove(val);
         nums.remove(nums.size() - );
         return true;
     }

     /** Get a random element from the set. */
     public int getRandom() {
         return nums.get( rand.nextInt(nums.size()) );
     }
 }

第二种情况：允许有重复数值。

用set来记录同一个值出现的位置，如果被删除的数不是最后一个，则和最后一个换一下位置，然后删除就可以了。

 public class RandomizedCollection {
     List<Integer> nums;
     Map<Integer, Set<Integer>> valueToPositionMap;
     Random rand;

     public RandomizedCollection() {
         nums = new ArrayList<>();
         valueToPositionMap = new HashMap<>();
         rand = new Random(System.currentTimeMillis());
     }

     /**
      * Inserts a value to the collection. Returns true if the collection did not
      * already contain the specified element.
      */
     public boolean insert(int val) {
         boolean isContained = valueToPositionMap.containsKey(val);
         if (!isContained) {
             valueToPositionMap.put(val, new HashSet<>());
         }
         valueToPositionMap.get(val).add(nums.size());
         nums.add(val);
         return !isContained;
     }

     /**
      * Removes a value from the collection. Returns true if the collection contained
      * the specified element.
      */
     public boolean remove(int val) {
         if (!valueToPositionMap.containsKey(val)) {
             return false;
         }
         if (!valueToPositionMap.get(val).contains(nums.size() - )) {
             int currPos = valueToPositionMap.get(val).iterator().next();
             int lastVal = nums.get(nums.size() - );
             valueToPositionMap.get(lastVal).remove(nums.size() - );
             valueToPositionMap.get(lastVal).add(currPos);
             valueToPositionMap.get(val).remove(currPos);
             valueToPositionMap.get(val).add(nums.size() - );
             nums.set(currPos, lastVal);
         }
         valueToPositionMap.get(val).remove(nums.size() - );
         if (valueToPositionMap.get(val).isEmpty()) {
             valueToPositionMap.remove(val);
         }
         nums.remove(nums.size() - );
         return true;
     }

     /** Get a random element from the collection. */
     public int getRandom() {
         return nums.get(rand.nextInt(nums.size()));
     }
 }