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Mastering Dictionary<TKey, TValue> in C# for Coding Interviews

#csharp #datastructures #interview #dictionary

🔑 Dictionary in C# — The Ultimate Tool for Fast Lookup

A Dictionary<TKey, TValue> is a powerful data structure that maps unique keys to values, allowing fast O(1) lookups and efficient data grouping.

✅ When to Use Dictionary

Use Case Why Use Dictionary
Map key to value Quick access with known key
Frequency count Count how many times items appear
Grouping elements Categorize data using keys
Implement cache or memoization Fast storage and retrieval

✍️ Declaring and Initializing 

// Declare and initialize
Dictionary<string, int> ageMap = new Dictionary<string, int>();

// Adding entries
ageMap["Alice"] = 25;
ageMap["Bob"] = 30;
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🔧 Common Operations 

// Add or update
ageMap["Charlie"] = 28;

// Check key existence
if (ageMap.ContainsKey("Alice"))
    Console.WriteLine(ageMap["Alice"]);

// Remove entry
ageMap.Remove("Bob");

// Loop through dictionary
foreach (var kvp in ageMap)
{
    Console.WriteLine($"{kvp.Key} => {kvp.Value}");
}
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🧪 Interview Example 1: Two Sum 

public int[] TwoSum(int[] nums, int target)
{
    var map = new Dictionary<int, int>();
    for (int i = 0; i < nums.Length; i++)
    {
        int complement = target - nums[i];
        if (map.ContainsKey(complement))
            return new int[] { map[complement], i };
        map[nums[i]] = i;
    }
    return new int[] { -1, -1 };
}
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🧪 Interview Example 2: Group Anagrams 

public IList<IList<string>> GroupAnagrams(string[] strs)
{
    var map = new Dictionary<string, List<string>>();

    foreach (var word in strs)
    {
        char[] chars = word.ToCharArray();
        Array.Sort(chars);
        string key = new string(chars);

        if (!map.ContainsKey(key))
            map[key] = new List<string>();

        map[key].Add(word);
    }

    return new List<IList<string>>(map.Values);
}
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📌 Summary

Feature Syntax Example
Declare new Dictionary<TKey, TValue>()
Add/Update dict[key] = value;
Exists Check dict.ContainsKey(key)
Remove dict.Remove(key)
Iterate foreach (var kvp in dict)

Next up: Stack — ideal for solving nested and reverse order problems like Valid Parentheses and backtracking tasks.

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ghost_engineer_2883a1ca0a profile image
Ghost Engineer •

try this if you get stuck during the interview. its an AI co-pilot that solves the questions for you so you can focus on the more important part of the interview, the communication part. its also a really good study tool: ghostengineer.com