[leetcode]380. Insert Delete GetRandom O(1)常数时间插入删除取随机值

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.

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 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

Solution1:  HashMap + ArrayList

1. use an ArrayList together with HashMap, making hashmap save key(item)->value(idx) 

2. To reverse item in O(1), avoiding traversal the whole arrayList in O(n) time, we swap the toDelete item with last item in the list 

3. Go through an example like this: 

   use HashMap to get removeIdx:

   Set the last item in the list as replaceValue, why the last item? In order to maintain other indices.

   Deal with HashMap: (1) put  (2)delete 

 Deal with List: (1) set  (2)delete

                                                                                      

 code:

class RandomizedSet {
    Map<Integer, Integer> map;
    List<Integer> list;
    Random r; // java 自带类

    /** Initialize your data structure here. */
    public RandomizedSet() {
        list = new ArrayList<>();
        map = new HashMap<>();
        r = new Random();    
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (map.containsKey(val)) {
            return false;
        }
        map.put(val, list.size());
        list.add(val);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        /**
        hashmap
        30 - 0
        40 - 1
        50 - 2
        60 - 3 
        
        list: 30 40  50 60 
               0   1  2  3
        **/
        if (!map.containsKey(val))     return false;
        int removeIdx = map.get(val);  // Idx: 1
        int replaceValue = list.get(list.size()-1); // replaceValue : 60 
        
        // deal with map
        map.put(replaceValue, removeIdx);
        map.remove(val);
        
        // deal with list
        list.set(removeIdx, replaceValue); 
        list.remove(list.size() -1);
      
        return true;     
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(r.nextInt(list.size()));
    }
}