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Virtual Algorithm Lab

Reverse a String: Out-of-Place

{ }
++ ++
0
0
new_s += s[i] — O(N) memory alloc
Out-of-Place Allocation System
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L
R

Out-of-Place Reversal

"hello" → "olleh" · iterate backwards to build a new string

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The Code Behind It
reverse_string.py 

    
In plain English: 

  


  
Theory & Fundamentals

  

    
String Immutability & O(N) Space

    
In languages like Python and Java, strings are immutable (they cannot be changed in-place). To reverse a string, we must allocate a completely new string in memory and build it character-by-character from the end of the original string. This requires O(N) auxiliary space.

  



  
Knowledge Check

  
  

    
Q1: Memory Allocation

    
If the original string has 10 characters, how many new character slots must be allocated in memory?

    

      
      
    

    

  


  

    
Q2: Iteration Direction

    
To build the reversed string sequentially, where does the loop pointer start?

    
    

  


  

    
Q3: Space Complexity

    
What is the auxiliary space complexity of reversing an immutable string by building a new one?