我的哈希表比二进制搜索慢

Question

我已经实现了二进制搜索,线性搜索和哈希表来比较每个时间复杂度.问题是,当我测量查找素数的时间时,我的哈希表比二进制搜索慢得多.以下是我的代码:

// Make the hash table 20 times the number of prime numbers
HashTable::HashTable(std::vector<int> primes)
{
    int tablesize = primes.size() * 20;
    table = new std::list<int>[tablesize];
    size = tablesize;
    for (auto &prime : primes)
        this->insert(prime);
}

// Hash function
int HashTable::hash(int key)
{
    return key % size;
}

// Finds element
int HashTable::find(int key)
{
    // Get index from hash
    int index = hash(key);

    // Find element
    std::list<int>::iterator foundelement = std::find(table[index].begin(), table[index].end(), key);


    // If element has been found return index
    // If not, return -1
    if (foundelement != table[index].end())
        return index;
    else
        return -1;
}



// Adds element to hashtable
void HashTable::insert(int element)
{
    // Get index from hash and insert the element
    int index = hash(element);
    table[index].push_back(element);
}

HashTable.h

#ifndef HASHTABLE_H
#define HASHTABLE_H

#include <list>
#include <iostream>
#include <vector>

class HashTable 
{
private:
    // Each position in Hashtable has an array of lists to store elements in case of collision
    std::list<int>* table;

    // Size of hashtable
    int size;

    // Hashfunction that returns the array location for the given key
    int hash(int key);

public:

    HashTable(int tablesize);
    HashTable(std::vector<int> primes);

    // Adds element to hashtable
    void insert(int element);

    // Deletes an element by key 
    void remove(int key);

    // Returns an element from hashtable for a given key
    int find(int key);

    // Displays the hashtable
    void printTable();

    // Display histogram to illustrate elements distribution
    void printHistogram();

    // Returns the number of lists in hash table
    int getSize();

    // Returns the total number of elements in hash table
    int getNumberOfItems();

    // De-allocates all memory used for the Hash Table.
    ~HashTable();
};

#endif

我已经尝试超过表格大小以消除碰撞,但我没有注意到任何差异.

Answer 1

这一切都与复杂性有关，二分搜索是 O(log n) ，并且您的查找是线性的，因此 O(n) ，在某一点上，当您有很多冲突时，这是最糟糕的。