Mastering Stack<T> in C# for Coding Interviews

[Lomanu4]

? Stack in C# — Solving Problems the LIFO Way


A Stack<T> is a Last-In-First-Out (LIFO) collection — the last element added is the first one removed. It's commonly used in parsing, expression evaluation, and backtracking.

When to Use Stack

Use Case	Why Use Stack
Reversing elements	Natural LIFO behavior
Nested structures / matching	e.g., parentheses, XML, recursion stack
Backtracking	Push state, pop to revert
Undo features	Store previous states

Declaring and Using Stack


var stack = new Stack<int>();

stack.Push(10); // Add to top
stack.Push(20);
int top = stack.Peek(); // Peek top (20)
stack.Pop(); // Remove top (20)
? Iterating Through Stack


foreach (int item in stack)
{
Console.WriteLine(item);
}
? Interview Example 1: Valid Parentheses


public bool IsValid(string s)
{
var stack = new Stack<char>();
var map = new Dictionary<char, char> {
{')', '('}, {']', '['}, {'}', '{'}
};

foreach (char c in s)
{
if (map.ContainsValue(c))
stack.Push(c);
else if (map.ContainsKey(c))
{
if (stack.Count == 0 || stack.Pop() != map[c])
return false;
}
}

return stack.Count == 0;
}
? Interview Example 2: Evaluate Reverse Polish Notation


public int EvalRPN(string[] tokens)
{
var stack = new Stack<int>();

foreach (var token in tokens)
{
if (int.TryParse(token, out int num))
stack.Push(num);
else
{
int b = stack.Pop();
int a = stack.Pop();
switch (token)
{
case "+": stack.Push(a + b); break;
case "-": stack.Push(a - b); break;
case "*": stack.Push(a * b); break;
case "/": stack.Push(a / b); break;
}
}
}

return stack.Pop();
}
? Summary

Feature	Syntax Example
Declare	new Stack<T>()
Push	stack.Push(value)
Pop	stack.Pop()
Peek	stack.Peek()
Count	stack.Count
Iterate	foreach (var item in stack)

Up next: Queue and Deque — perfect for BFS traversal and sliding window problems!

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