How to Efficiently Retrieve Multiple Keys Associated with the Same Value in a Dictionary?

How to retrieve multiple keys with specified values ​​from a generic dictionary

.NET generic dictionaries provide an efficient way to retrieve the value associated with a key, as shown in the following code:

<code class="language-csharp">Dictionary<int, string> greek = new Dictionary<int, string>();
greek.Add(1, "Alpha");
greek.Add(2, "Beta");
string secondGreek = greek[2];  // Beta</code>

However, retrieving the key associated with a given value is not as simple as with a generic dictionary, since a generic dictionary only stores a single key-value pair for each unique key. This can create challenges when you need to find all keys that correspond to a specific value, especially when multiple keys may map to the same value.

To solve this problem, the following code implements a two-way dictionary that allows keys and values ​​to be retrieved in a generic way:

<code class="language-csharp">using System;
using System.Collections.Generic;
using System.Text;

class BiDictionary<TFirst, TSecond>
{
    IDictionary<TFirst, IList<TSecond>> firstToSecond = new Dictionary<TFirst, IList<TSecond>>();
    IDictionary<TSecond, IList<TFirst>> secondToFirst = new Dictionary<TSecond, IList<TFirst>>();

    // ... (方法和属性的实现) ...
}</code>

This two-way dictionary maintains two sets of key-value mappings: one is a list from the first key to the second value, and the other is a list from the second value to the first key. This allows efficient bi-directional retrieval of keys and values.

For example, consider a dictionary of Greek words, where each Greek letter is assigned a numerical value. Using a two-way dictionary you can easily get the Greek letter corresponding to a given numerical value like this:

<code class="language-csharp">BiDictionary<int, string> greek = new BiDictionary<int, string>();
greek.Add(1, "Alpha");
greek.Add(2, "Beta");
greek.Add(5, "Beta");

// 检索对应于值“Beta”的希腊字母
IList<int> betaKeys = greek.GetBySecond("Beta");

// 显示结果
Console.WriteLine("Keys for \"Beta\":");
foreach (int key in betaKeys)
{
    Console.WriteLine(key);
}</code>

This method handles duplicate values ​​elegantly by returning a list of keys corresponding to the specified value. It demonstrates the versatility of two-way dictionaries when dealing with dictionaries in which multiple keys may map to the same value.

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