381. Insert Delete GetRandom O(1)

原题

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();

代码实现

/* ============================================================================== * 功能描述：IDG   * 创 建 者：vuefine * 创建日期：2017/5/11  * ==============================================================================*/using System;using System.Collections.Generic;using System.Data.SqlTypes;using System.Linq;using System.Text;using System.Threading;namespace IDGSln{    /// <summary>    /// IDG    /// </summary>    public class RandomizedCollection    {        //insert remove getRandom        private Dictionary<int, List<int>> _dict;  //key: value, Value: index collection        private List<int> _valList; //element of value list, for it can get the O(1) visit        public RandomizedCollection()        {            _dict = new Dictionary<int, List<int>>();            _valList = new List<int>();        }        public bool Insert(int val)        {            if (!_dict.ContainsKey(val))            {                _dict.Add(val, new List<int> { _valList.Count });                _valList.Add(val);                return true;            }            _dict[val].Add(_valList.Count);            _valList.Add(val);            return false;        }        public bool Remove(int val)        {            if (!_dict.ContainsKey(val)) return false;            List<int> removeIndexCollection = _dict[val]; //_dict.Values: element index collection            swapToLast(removeIndexCollection[removeIndexCollection.Count-1]);                        _valList.Remove(val);            if (removeIndexCollection.Count == 1)                _dict.Remove(val);            else _dict[val].Remove(val);            return true;        }        //i-待移除元素的索引        //将待移除元素移到_valList的末尾，修改原来末尾元素在字典中的键值        private void swapToLast(int i)        {            int lastIndex = _valList.Count - 1;            if (i > lastIndex) return;            int lastVal = _valList[lastIndex];            int tmpVal = _valList[i];            _valList[i] = lastVal; // the key is to prove " i < lastIndex" !!!            _valList[lastIndex] = tmpVal;            //this is important, to update the last index value in dictionary            if(i!=lastIndex) //i 不是末尾元素               _dict[lastVal].Add(i);        }        public int GetRandom()        {            long tick = DateTime.Now.Ticks;            Random ran = new Random((int)(tick & 0xffffffffL) | (int)(tick >> 32));            int randnum = ran.Next(_valList.Count);            if (_valList.Count == 0) return -1;            Thread.Sleep(20);            return _valList[randnum];        }    }}

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