Mastering HashSet in C# for Coding Interviews

[Lomanu4]

? HashSet in C# — A Must-Know Tool for Uniqueness and Fast Lookups


HashSet<T> is one of the most versatile and commonly used data structures in coding interviews. It allows constant-time lookup and guarantees unique elements only.

When to Use HashSet

Use Case	Why Use HashSet
Check for duplicates	Fast Contains() check
Track seen items	O(1) add/lookup
Unique collections	Prevents duplicates automatically
Set operations (union/intersect)	Built-in set logic

Declaring and Using HashSet


var seen = new HashSet<int>();

seen.Add(5); // true
seen.Add(5); // false (duplicate)

bool exists = seen.Contains(5); // true
seen.Remove(5); // removes 5
? Looping Through HashSet


foreach (int num in seen)
{
Console.WriteLine(num);
}
? Interview Example 1: Detect Duplicates


public bool ContainsDuplicate(int[] nums)
{
var set = new HashSet<int>();
foreach (int num in nums)
{
if (!set.Add(num)) return true;
}
return false;
}
? Interview Example 2: Longest Substring Without Repeating Characters


public int LengthOfLongestSubstring(string s)
{
var set = new HashSet<char>();
int left = 0, maxLen = 0;

for (int right = 0; right < s.Length; right++)
{
while (set.Contains(s

))
{
set.Remove(s

);
left++;
}

set.Add(s

);
maxLen = Math.Max(maxLen, right - left + 1);
}

return maxLen;
}
Set Operations


var a = new HashSet<int>() { 1, 2, 3 };
var b = new HashSet<int>() { 2, 3, 4 };

a.IntersectWith(b); // a = {2, 3}
a.UnionWith(b); // a = {1, 2, 3, 4}
a.ExceptWith(b); // a = {1}
? Summary

Feature	Syntax Example
Declare	new HashSet<int>()
Add	set.Add(value)
Check Exists	set.Contains(value)
Remove	set.Remove(value)
Set Ops	UnionWith(), IntersectWith()

Next up: Dictionary — perfect for counting, mapping, and solving problems like Two Sum and Group Anagrams.

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