Reverse a Stack using C#

A stack is a Last-In-First-Out (LIFO) data structure in C#. To reverse a stack, we can use another stack and leverage the LIFO property by popping elements from the original stack and pushing them into a new stack.

The System.Collections.Stack class provides Push() and Pop() methods that make stack reversal straightforward.

Syntax

Following is the basic syntax for creating and reversing a stack −

Stack originalStack = new Stack();
Stack reversedStack = new Stack();

while (originalStack.Count != 0) {
   reversedStack.Push(originalStack.Pop());
}

How Stack Reversal Works

The reversal process works by exploiting the LIFO nature of stacks. When we pop elements from the original stack and push them into a new stack, the order gets reversed.

Using Another Stack for Reversal

Example

using System;
using System.Collections;

public class Program {
   public static void Main(string[] args) {
      Stack st = new Stack();
      Stack rev = new Stack();
      
      st.Push('P');
      st.Push('Q');
      st.Push('R');
      
      Console.WriteLine("Original stack:");
      foreach(char c in st) {
         Console.Write(c + " ");
      }
      Console.WriteLine();
      
      while (st.Count != 0) {
         rev.Push(st.Pop());
      }
      
      Console.WriteLine("Reversed stack:");
      foreach(char c in rev) {
         Console.Write(c + " ");
      }
   }
}

The output of the above code is −

Original stack:
R Q P 
Reversed stack:
P Q R 

Using Recursion for In-Place Reversal

We can also reverse a stack in-place using recursion without using an additional stack −

Example

using System;
using System.Collections;

public class Program {
   public static void ReverseStack(Stack st) {
      if (st.Count <= 1) return;
      
      object temp = st.Pop();
      ReverseStack(st);
      InsertAtBottom(st, temp);
   }
   
   public static void InsertAtBottom(Stack st, object item) {
      if (st.Count == 0) {
         st.Push(item);
         return;
      }
      
      object temp = st.Pop();
      InsertAtBottom(st, item);
      st.Push(temp);
   }
   
   public static void Main(string[] args) {
      Stack st = new Stack();
      st.Push('A');
      st.Push('B');
      st.Push('C');
      
      Console.WriteLine("Original stack:");
      foreach(char c in st) {
         Console.Write(c + " ");
      }
      Console.WriteLine();
      
      ReverseStack(st);
      
      Console.WriteLine("Reversed stack:");
      foreach(char c in st) {
         Console.Write(c + " ");
      }
   }
}

The output of the above code is −

Original stack:
C B A 
Reversed stack:
A B C 

Comparison of Methods

Conclusion

Reversing a stack in C# can be accomplished using another stack for O(n) time complexity, or recursively for in-place reversal. The additional stack method is more efficient and commonly used, while recursion provides an elegant solution without extra data structures.