381. Insert Delete GetRandom O(1) - Duplicates allowed允许重复的设计1数据结构

［抄题］：

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

Note: Duplicate elements are allowed.

insert(val): Inserts an item val to the collection.
remove(val): Removes an item val from the collection if present.
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］：

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
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()
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();

 */

巴特西

381. Insert Delete GetRandom O(1) - Duplicates allowed允许重复的设计1数据结构

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