Add Permutation

[moonjs0113]

Permutation

Description

This is permutation implementation in Swift.

Feature

• It can determine the length of the permutation.
• Returns nil if the count is out of valid range.

Code

func permutations<T>(data: [T], count: Int) -> [[T]]? {
    guard data.count >= count && count >= 0 else { return nil }
    var result: [[T]] = []
    var buffer: [T] = []
    var visited = Array(repeating: false, count: data.count)

    func nextElement() {
        if buffer.count == count {
            result.append(buffer)
            return
        }
        for i in 0..<data.count {
            if !visited[i] {
                visited[i] = true
                buffer.append(data[i])
                nextElement()
                buffer.removeLast()
                visited[i] = false
            }
        }
    }
    nextElement()
    return result
}