Reverse String

Given a slice of bytes representing a string, reverse it in-place.

Function signature

func ReverseString(s []byte)

Inputs / outputs

• s: byte slice to reverse.
• Return: nothing; modify s in-place.

Example

s = ['h','e','l','l','o']
After reverse: ['o','l','l','e','h']

Constraints

• 0 <= len(s) <= 100_000

Core idea (near-solution)

Use two pointers from both ends and swap until they cross.

Invariant: elements outside [l, r] are already in their final positions.

Algorithm (step-by-step)

1. Set l = 0, r = len(s)-1.
2. While l < r:
   • Swap s[l] and s[r].
   • Increment l, decrement r.
3. Done.

Correctness sketch

• Invariant: elements outside [l, r] are already in their final positions.
• Each step moves at least one pointer, shrinking the remaining search space.
• The invariant ensures elements outside the active window are already handled correctly.
• When pointers meet or cross, all candidates have been considered.

Trace (hand walkthrough)

Example input:
s = ['h','e','l','l','o']
After reverse: ['o','l','l','e','h']

Walk:
- l=0,r=4: swap h and o -> [o,e,l,l,h]
- l=1,r=3: swap e and l -> [o,l,l,e,h], done

Pitfalls to avoid

• Forgetting to handle empty slices.
• Confusing byte reversal with Unicode rune reversal (this function operates on bytes).

Complexity

• Time: O(n).
• Extra space: O(1).

Implementation notes (Go)

• []byte reversal is correct for ASCII and raw bytes; Unicode requires rune handling.