Design a data structure that supports all following operations in average 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.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 1 is the only number in the set, getRandom always return 1.randomSet.getRandom();

最先想到是用double LinkedList+Map, 没必要，arraylist+map就够了；另外取random的方法还有，rand.nextInt(int n) returns an integer in the range [0, n)

java.util.Random rand = new java.util.Random();

return nums.get( rand.nextInt(nums.size()) );

1 public class RandomizedSet { 2     HashMap
     
       map; 3     ArrayList
      
        arr; 4  5     /** Initialize your data structure here. */ 6     public RandomizedSet() { 7         map = new HashMap
       
        (); 8         arr = new ArrayList
        
         (); 9     }10     11     /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */12     public boolean insert(int val) {13         if (map.containsKey(val)) return false;14         arr.add(val);15         map.put(val, arr.size()-1);16         return true;17     }18     19     /** Removes a value from the set. Returns true if the set contained the specified element. */20     public boolean remove(int val) {21         if (!map.containsKey(val)) return false;22         int lastItem = arr.get(arr.size()-1);23         if (val != lastItem) {24             int index = map.get(val);25             arr.set(index, lastItem);26             map.put(lastItem, index);27         }28         arr.remove(arr.size()-1);29         map.remove(val);30         return true;31     }32     33     /** Get a random element from the set. */34     public int getRandom() {35         return arr.get((int)(Math.random()*arr.size()));36     }37 }38 39 /**40  * Your RandomizedSet object will be instantiated and called as such:41  * RandomizedSet obj = new RandomizedSet();42  * boolean param_1 = obj.insert(val);43  * boolean param_2 = obj.remove(val);44  * int param_3 = obj.getRandom();45  */