Leetcode #38: "The count-and-say sequence is the sequence of integers with the first five terms as following: 1) '1' 2) '11' 3) '21' 4. '1211' 5. '111221' 1 is read off as 'one 1' or 11. 11 is read off as 'two 1s' or 21. 21 is read off as 'one 2, then one 1' or 1211. Given an integer n where 1 ≤ n ≤ 30, generate the nth term of the ... sequence."

CountAndSay.swift

class Solution {
    func countAndSay(_ n: Int) -> String {
        guard n > 0 else {
            fatalError("Positive integers only!")
        }
        var last = "1"
        if n == 1 {
            return last
        }
        for _ in 2...n {
            last = describeCharsIn(string: last)
        }
        return last
    }
    
    private func describeCharsIn(string: String) -> String {
        guard !string.isEmpty else {
            return ""
        }
        guard string.count > 1, let first = string.first else {
            return "1\(string)"
        }
        
        var outputString = ""
        var previous = first
        var countOfPrevious = 1
        let finalIndex = string.count - 1
        
        var currentIndex = 0
        for current in string {
            if currentIndex == 0 {
                currentIndex += 1
                continue
            }
            
            let isFinalIndex = (currentIndex == finalIndex)
            let isSameAsPrevious = (current == previous)
            if isSameAsPrevious {
                countOfPrevious += 1
            }
            
            if isFinalIndex && isSameAsPrevious {
                outputString += "\(countOfPrevious)\(current)"
            }
            else if isFinalIndex && !isSameAsPrevious {
                outputString += "\(countOfPrevious)\(previous)"
                outputString += "1\(current)"
            }
            else if !isFinalIndex && !isSameAsPrevious {
                outputString += "\(countOfPrevious)\(previous)"
                countOfPrevious = 1
            }
            
            previous = current
            currentIndex += 1
        }
        return outputString
    }
}

/* Current state of this solution on Leetcode (a 97% reduction in runtime!):
-------------------------------------------------------------------------------------
Runtime: 12 ms, faster than 64.89% of Swift online submissions for Count and Say.
Memory Usage: 20.9 MB, less than 100.00% of Swift online submissions for Count and Say.
-------------------------------------------------------------------------------------

Previously:
-------------------------------------------------------------------------------------
Runtime: 336 ms, faster than 5.33% of Swift online submissions for Count and Say.
Memory Usage: 21 MB, less than 100.00% of Swift online submissions for Count and Say.
-------------------------------------------------------------------------------------
*/