Reverse words in a sentence - Swift

Question

How to perform reversing of all words in a sentence.

Example

let str = \"Hello playground\"

Result should be like \"olleH dnuorg

Answer 1

Here's the typical functional programming approach (also posted by @Saranjith)

let result = str
    .components(separatedBy: " ")
    .map { $0.reversed() }
    .joined()

Complexity

First of all lets define

• n: the number of characters in the input string
• k: the number of words in the input string (of course k<=n)

⏳ Time complexity

Now lets look at the time complexity of each piece of our code

.components(separatedBy: " ")

This instructions need to go through the entire string so O(n)

.map { $0.reversed() }

Here each word is reversed. So if we have k words we have a time complexity of

O(m0) + O(m1) + ... + O(mk-1)

where mi is the length of the i-th word. However the sum of the length of all the words is <= n so we can say that

O(m0) + O(m1) + ... + O(mk-1) <= O(n)

.joined()

Finally we have k words which need to be joined togheter. This can be done in O(k) that, again, is <= O(n).

Wrap up

let result = str
    .components(separatedBy: " ") // O(n)
    .map { $0.reversed() } // O(m0) + O(m1) + ... + O(mk-1) <= O(n)
    .joined() // O(k) <= O(n)

Time complexity = O(n) + O(n) + O(n) = O(n)