Python Algorithm Q2

Q2 Reverse-String

---

Q

Write a function that reverses a string. The input string is given as an array of characters s.

You must do this by modifying the input array in-place with O(1) extra memory.

Sample Input

Input: s = [“h”,”e”,”l”,”l”,”o”] Output: [“o”,”l”,”l”,”e”,”h”]

Input: s = [“H”,”a”,”n”,”n”,”a”,”h”] Output: [“h”,”a”,”n”,”n”,”a”,”H”]

Constraints

• 1 <= s.length <= 10⁵
• s[i] is a printable ascii character.

1. My Solution

class Solution:
    def reverseString(self, s: List[str]) -> None:
        for i in range(len(s)//2):
            a: str = s[i] #save first
            print(a)
            s[i]=s[-(i+1)]
            print(s[i])
            s[-(i+1)] = a

#for testing
s = Solution()
s.reverseString(["a","b","c","d"])

Simple Concept. Just saved the first element to temporary ‘a’ variable, and put the last element(s[-1]) to the first one(s[0]). Keep changing while the loop meets the center. I chose ‘len(s)//2’, since residue is useless. That’s because the center element don’t need to be changed.

In my solution, I have to be aware of the type of ‘s’. I put s.reverseString("hello") in my first try.
In this case, the type of ‘s’ become just string. not list comprise of string.
Therefore the error ‘TypeError: ‘str’ object does not support item assignment’ occurs. Be Careful! Python’s type hint is just the ‘hint’. It doesn’t define the type of the object.

2. Given Solution #1

def reverseString(self, s: List[str]) -> None:
    left, right = 0, len(s) - 1
    while left < right:
        s[left], s[right] = s[right], s[left]
        left += 1
        right -= 1

No Commentary is needed for this solution.

3. Given Solution #2

According to the book, this is the ‘Pythonic’ way.

def reverseString(self, s: List[str]) -> None:
    s.reverse()

Wow… the method ‘reverse’ is only provided when using the list.

When if the input is the string, U can use the string slicing method like s = s[::-1]

Overall Review

The solution that called ‘Pythonic’ way makes me shocked. Python library is literally amazing and contains incountable method. I should learn more to use python better.