String Reversal- Coding Problem

A string is given as input. We have to print the reversed string.
For example, the string is “Hi how are you?” and the output should be “?ouy era woh iH”

The first solution is we can create a new array and append the characters of the original array, one by one from the end to the beginning.

def simple_reverse(string):
    new_string = []
    for i in range(len(string)-1, -1, -1): 
#The for loop runs from the last element to the first element of the original string
        new_string.append(string[i]) 
    return ''.join(new_string) 

string = "Hello"
print(simple_reverse(string))

Since we only have to traverse the string once, the time complexity is O(n)
But since we are also creating a new array of the same size , the space complexity is also O(n).

A smarter way to do this , can be taking a pair of elements from either end of the string and swapping them
We have start at both the ends and continue swapping pairs till the middle of the string
This way we can avoid having to create a new array and save on space complexity while keeping time complexity at O(n)

def swap(string, a, b): #Function which swaps two characters of a string
    string = list(string)
    temp = string[a]
    string[a] = string[b]
    string[b] = temp
    return ''.join(string)

def smarter_reverse(string):
    for i in range(len(string)//2):
        string = swap(string, i, len(string)-i-1)
    return string

print(smarter_reverse(string))

Apart from these, some built-in functions that can be used to reverse a string are as follows:

string1 = 'abcde'
string2 = reversed(string1)
print(''.join(string2))

list1 = list(string1)
list1.reverse()
print(''.join(list1))

#Both these methods are of O(n) time complexity