🧱 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!
Top comments (2)
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