Reverse a String

You are given a list of characters s. Reverse the list in-place. Do not return anything; modify the list directly.

Answer

Method 1, Trivial slicing:

class Solution:
    def reverseString(self, s: List[str]) -> None:
        reverse_string = s[::-1]
        for i in range(len(reverse_string)):
            s[i] = reverse_string[i]

Method 2, Manual reverse: Build a reversed list manually by iterating from the end, then copy values back.

class Solution:
    def reverseString(self, s: List[str]) -> None:
        r = len(s) - 1
        reverse_s = []

        while r >= 0:
            reverse_s.append(s[r])
            r -= 1

        for i in range(len(reverse_s)):
            s[i] = reverse_s[i]

Method 3, Two pointers: Swap the left and right characters until the pointers meet.

class Solution:
    def reverseString(self, s: List[str]) -> None:
        l, r = 0, len(s) - 1

        while l <= r:
            s[l], s[r] = s[r], s[l]
            r -= 1
            l += 1

Programming lesson:

Doing s = reverse.copy() does not work for in-place modification because it only reassigns the local variable s to a new list. The original list (the one outside the function) remains unchanged. However, writing s[i] = ... does modify the input list in-place, because we are updating the existing list’s elements.
In Method 3, the swap
Python
```
s[l], s[r] = s[r], s[l]
```
happens simultaneously. In Python, the right-hand side is evaluated first (creating a temporary tuple), and then both assignments occur together. This prevents overwriting.
If done in two separate lines overwriting would happen.