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

[Lomanu4]

? 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;
? 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}");
}
? 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;
if (map.ContainsKey(complement))
return new int[] { map[complement], i };
map[nums] = i;
}
return new int[] { -1, -1 };
}
? 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);
}
? 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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