Q.Program 8-3: Write a program to input a string from the user and print it in the reverse order without creating a new string.

String Reversal In Place

The Intuition

Imagine you have a word written on a whiteboard: C A T. Now you want to reverse it so it reads T A C. The most natural way is to swap the first and last letters, then move inward.

Swap C and T: T A C — done. That took one swap for a 3-letter word.

Now try B E A R. Swap B and R: R E A B. Swap E and A: R A E B. Two swaps for a 4-letter word.

Notice the pattern: you never need extra space. You're just swapping pairs of characters on the same board. That's the core idea of in-place reversal — you transform the string using only the memory it already occupies, without copying it to a new location.

The Precise Statement

String reversal in place means reversing the order of characters in a string using only $O(1)$ extra space (a constant amount of memory, like a few variables), and modifying the original string directly.

The algorithm works like this:

1. Let the string have length $n$.
2. Place one pointer at index $0$ (left) and another at index $n-1$ (right).
3. While left < right:
   • Swap the characters at left and right.
   • Move left one step right, right one step left.

That's it. After the loop, the string is reversed.

Why It Works

Each swap puts a character from the left half into its mirror position on the right half, and vice versa. The pointers meet in the middle, and by then every pair has been swapped exactly once.

For a string of length $n$, the number of swaps is $\lfloor n/2 \rfloor$. If $n$ is odd, the middle character stays where it is — no swap needed.

A Concrete Example

Reverse "hello":

Initial:  h e l l o
          L       R

Swap L and R:  o e l l h …