哈希表是一种可以使用关键字（key）来快速查询值的数据结构。每个数据都有它自己的关键字, 通过哈希函数处理之后得到一个整数，叫索引值（index）。
想要查找某个关键字对应得值，需要查找哈希函数处理后对应得索引值，以此得到想要得值。
哈希函数简单来说就是取模运算。所以肯定会有不同的关键字得到了相同的索引值的情况，这种情况叫冲突。解决方法有许多，简单来说就是在相同的索引附近找一个空的地方把数据放进去。
哈希表的优点是可以快速查询，平均查找时, 以及关键字能够取很多的数据类型，只要能够用哈希函数。
缺点是在最坏的情况下，需要的时间为；关键字不是按顺序排列的，如果想找个最小或者最大的key对应的值就需要全都找一遍；单项查询，找某个关键字对应的值只需要， 找某个值对应的关键字就要全都找一遍。

C++

// C++ program to demonstrate functionality of unordered_map 
#include <iostream> 
#include <unordered_map> 
using namespace std; 

int main() 
{ 
    // Declaring umap to be of <string, double> type 
    // key will be of string type and mapped value will 
    // be of double type 
    unordered_map<string, double> umap; 
  
    // inserting values by using [] operator 
    umap["PI"] = 3.14; 
    umap["root2"] = 1.414; 
    umap["root3"] = 1.732; 
    umap["log10"] = 2.302; 
    umap["loge"] = 1.0; 

    // inserting value by insert function 
    umap.insert(make_pair("e", 2.718)); 
     
    string key = "PI"; 
    // If key not found in map iterator to end is returned 
    if (umap.find(key) == umap.end()) 
        cout << key << " not found\n\n"; 
    // If key found then iterator to that key is returned 
    else
        cout << "Found " << key << endl;

     // iterating over all value of umap 
    unordered_map<string, double>:: iterator itr; 
    cout << "\nAll Elements : \n"; 
    for (itr = umap.begin(); itr != umap.end(); itr++) 
    { 
        // itr works as a pointer to pair<string, double> 
        // type itr->first stores the key part  and 
        // itr->second stores the value part 
        cout << itr->first << "  " << itr->second << endl; 
     } 
} 

Output:
Found PI

All Elements : 
loge  1
e  2.718
log10  2.302
root3  1.732
PI  3.14
root2  1.414

一个应用: 给定一个字符串，统计里面每个字出现的频率。

// C++ program to find freq of every word using 
// unordered_map 
#include <bits/stdc++.h> 
using namespace std; 
  
// Prints frequencies of individual words in str 
void printFrequencies(const string &str) 
{ 
    // declaring map of <string, int> type, each word 
    // is mapped to its frequency 
    unordered_map<string, int> wordFreq; 
  
    // breaking input into word using string stream 
    stringstream ss(str);  // Used for breaking words 
    string word; // To store individual words 
    while (ss >> word) 
        wordFreq[word]++; 
  
    // now iterating over word, freq pair and printing 
    // them in <, > format 
    unordered_map<string, int>:: iterator p; 
    for (p = wordFreq.begin(); p != wordFreq.end(); p++) 
        cout << "(" << p->first << ", " << p->second << ")\n"; 
} 
  
// Driver code 
int main() 
{ 
    string str = "geeks for geeks geeks quiz "
                 "practice qa for"; 
    printFrequencies(str); 
    return 0; 
} 

Output:
(qa, 1)
(quiz, 1)
(practice, 1)
(geeks, 3)
(for, 2)

代码来源：
GeeksforGeeks--unordered_map in C++ STL

Python

在python里，字典数据类型（Dictionary）用的就是哈希表。

# Declare a dictionary 
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}

# Accessing the dictionary with its key
print "dict['Name']: ", dict['Name']
print "dict['Age']: ", dict['Age']

Output:
dict['Name']:  Zara
dict['Age']:  7

# Add new entry
dict['School'] = "DPS School"; 

 # remove entry with key 'Name'
del dict['Name'];
# remove all entries in dict
dict.clear();     
 # delete entire dictionary
del dict ;       

应用：
给定一个整数数组 nums 和一个目标值 target，在该数组中找出和为目标值的那两个整数，并返回他们的数组下标。

def twoSum(self, nums: List[int], target: int) -> List[int]:
    # Declare an empty dictionary
    dic = {}
    for idx, value in enumerate(nums):
        if target - value in dic:
            return [dic[target-value],idx]
        dic[value] = idx