Mastering List<T> in C# for Coding Interviews

[Lomanu4]

? List in C# — The Dynamic Array You Need to Know


List<T> is the most flexible and commonly used collection in C#. It’s a dynamic array that automatically resizes and provides indexed access.

When to Use List

Use Case	Why Use List
Need dynamic resizing	Automatically expands as needed
Index-based access	Like arrays, but flexible
Frequent insert/remove	Especially at the end
Storing objects or models	Generic and powerful

Declaring & Initializing List


// Create an empty list
List<int> numbers = new List<int>();

// Initialize with values
List<string> names = new List<string> { "Alice", "Bob", "Charlie" };
? Adding, Removing, and Accessing Elements


numbers.Add(10); // Add to end
numbers.AddRange(new int[] { 20, 30 }); // Add multiple

numbers.Remove(20); // Remove by value
numbers.RemoveAt(0); // Remove by index

int first = numbers[0]; // Access
numbers[1] = 100; // Update
? Iterating Through List


for (int i = 0; i < names.Count; i++)
{
Console.WriteLine(names);
}

foreach (string name in names)
{
Console.WriteLine(name);
}
? Searching and Sorting


bool exists = names.Contains("Bob"); // true
int index = names.IndexOf("Charlie"); // 2

names.Sort(); // Sort alphabetically
names.Reverse(); // Reverse order
? Interview Example: Merge Intervals (List of Lists)


public int[][] Merge(int[][] intervals)
{
if (intervals.Length <= 1) return intervals;

Array.Sort(intervals, (a, b) => a[0].CompareTo(b[0]));
var merged = new List<int[]>();
var current = intervals[0];

foreach (var interval in intervals)
{
if (interval[0] <= current[1])
current[1] = Math.Max(current[1], interval[1]);
else
{
merged.Add(current);
current = interval;
}
}
merged.Add(current);

return merged.ToArray();
}
? Summary

Feature	Syntax Example
Declare	List<int> list = new List<int>();
Add	list.Add(value), list.AddRange(...)
Access/Update	list, list = value

[TR]
[TD]Remove[/TD]
[TD] list.Remove(value), list.RemoveAt(i) [/TD]
[/TR]
[TR]
[TD]Sort/Reverse[/TD]
[TD] list.Sort(), list.Reverse() [/TD]
[/TR]

Up next: Dictionary — your go-to structure for frequency maps and lookup-based problems like Two Sum and Group Anagrams.

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