# 27 O(n²) Sorting Challenges Written by Kelvin Lau

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### Challenge 1: Group elements

Given a collection of `Equatable` elements, bring all instances of a given value in the array to the right side of the array.

### Challenge 2: Find a duplicate

Given a collection of `Equatable` (and `Hashable`) elements, return the first element that is a duplicate in the collection.

### Challenge 3: Reverse a collection

Reverse a collection of elements by hand. Do not rely on the `reverse` or `reversed` methods.

## Solutions

### Solution to Challenge 1

The trick to this problem is to control two references to manage swapping operations. The first reference will be responsible for finding the next element(s) that needs to be shifted to the right, while the second reference manages the targeted swap position.

``````extension MutableCollection
where Self: BidirectionalCollection, Element: Equatable {

mutating func rightAlign(value: Element) {
var left = startIndex
var right = index(before: endIndex)

while left < right {
while self[right] == value {
formIndex(before: &right)
}
while self[left] != value {
formIndex(after: &left)
}

guard left < right else {
return
}
swapAt(left, right)
}
}
}
``````

### Solution to Challenge 2

Finding the first duplicated element is quite straightforward. You use a `Set` to keep track of the elements you’ve encountered so far.

``````extension Sequence where Element: Hashable {

var firstDuplicate: Element? {
var found: Set<Element> = []
for value in self {
if found.contains(value) {
return value
} else {
found.insert(value)
}
}
return nil
}
}
``````

### Solution to Challenge 3

Reversing a collection is also quite straightforward. Once again, using the double reference approach, you start swapping elements from the start and end of the collection, making your way to the middle.

``````extension MutableCollection
where Self: BidirectionalCollection {

mutating func reverse() {
var left = startIndex
var right = index(before: endIndex)

while left < right {
swapAt(left, right)
formIndex(after: &left)
formIndex(before: &right)
}
}
}
``````

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