381. Insert Delete GetRandom O(1)

［抄题］：

Design a data structure that supports all following operations in average O(1) time.

Note: Duplicate elements are allowed.

1. insert(val): Inserts an item val to the collection.
2. remove(val): Removes an item val from the collection if present.
3. getRandom: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains.

Example:

// Init an empty collection.
RandomizedCollection collection = new RandomizedCollection();

// Inserts 1 to the collection. Returns true as the collection did not contain 1.
collection.insert(1);

// Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].
collection.insert(1);

// Inserts 2 to the collection, returns true. Collection now contains [1,1,2].
collection.insert(2);

// getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.
collection.getRandom();

// Removes 1 from the collection, returns true. Collection now contains [1,2].
collection.remove(1);

// getRandom should return 1 and 2 both equally likely.
collection.getRandom();

 [暴力解法]：

时间分析：

空间分析：

 [优化后]：

时间分析：

空间分析：

［奇葩输出条件］：

［奇葩corner case］：

如果hashmap的value为空，则必须把key也删掉

［思维问题］：

［一句话思路］：

用set存location, 然后还是先移动 后删除

［输入量］：空： 正常情况：特大：特小：程序里处理到的特殊情况：异常情况（不合法不合理的输入）：

［画图］：

［一刷］：

1. 提前判断存在性还是需要的，因为如果不存在时，需要初始化一个hashset
2. map中存的是nums数组的大小，表示目前元素插入成为了新数组最后一位：nums.size()
3. nums.remove(index，不是内容)

［二刷］：

［三刷］：

［四刷］：

［五刷］：

  [五分钟肉眼debug的结果]：

［总结］：

hashset可以分区

［复杂度］：Time complexity: O() Space complexity: O()

［英文数据结构或算法，为什么不用别的数据结构或算法］：

［算法思想：递归/分治/贪心］：

［关键模板化代码］：

［其他解法］：

［Follow Up］：

［LC给出的题目变变变］：

 [代码风格] ：

class RandomizedCollection {

    /** Initialize your data structure here. */
    HashMap<Integer, Set<Integer>> map;
    ArrayList<Integer> nums;
    java.util.Random rand = new java.util.Random();
    
    public RandomizedCollection() {
        map = new HashMap<Integer, Set<Integer>>();
        nums = new ArrayList<>();
    }
    
    /** 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 = map.containsKey(val);
        if (!contain) map.put(val, new HashSet<Integer>());
        
        map.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 = map.containsKey(val);
        if (!contain) return false;
        
        int loc = map.get(val).iterator().next();
        //renew location
        if (loc < nums.size() - 1) {
            int lastone = nums.get(nums.size() - 1);
            nums.set(loc, lastone);
            map.get(lastone).add(loc);
            map.get(lastone).remove(nums.size() - 1);
        }
        
        //remove, if key is empty
        nums.remove(nums.size() - 1);
        map.get(val).remove(loc);
        if (map.get(val).isEmpty()) map.remove(val);
        
        //return 
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        return nums.get(rand.nextInt(nums.size()));
    }
}

/**
 * Your RandomizedCollection object will be instantiated and called as such:
 * RandomizedCollection obj = new RandomizedCollection();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */