WIP for polish notation

polish.swift

//: Playground - noun: a place where people can play

import UIKit

//"2+3+4"
//"234++"



func polish(input: String) -> String {
    
    if input.characters.count <= 1 {
        return "broke"
    }
    
    let count = input.characters.count
    let delta = (count == 2 || count == 3) ? 1 : 2
    let head = String(input[input.startIndex])
    let tail = input[input.startIndex.advancedBy(delta)..<input.endIndex]
    switch count {
    case 2:
        return tail + String(head)
    case 3:
        return head + polish(tail)
    default:
        //if second character is *, the head
//        we're looking instead at the second to last operation
        let secondTolastIndex = input.endIndex.advancedBy(-4)
        if input[secondTolastIndex] == "+" {
            return String(input[input.startIndex])
                + polish(tail)
                + String(input[input.startIndex.advancedBy(1)])
        } else if input[secondTolastIndex] == "*" {
            let head = String(input[input.startIndex..<input.startIndex.advancedBy(3)])
            let tail = input[input.startIndex.advancedBy(3)..<input.endIndex]
            return polish(head) + polish(tail)
        }
        
        return String(input[input.startIndex.advancedBy(1)]) + "input: \(input)"
    }
}

//if the second character is * then the head is 

//2 * 3 + 4 - 23*4+
//2 + 3 * 4 - 234*+
//2 + 3 * 4 + 8 - 234*+8+



let input = "2+3*4"
let head = input[input.startIndex..<input.startIndex.advancedBy(2)]
let tail = input[input.endIndex.advancedBy(-4)]
//tail + String(head)
//let head = input[0]
//let tail = input[1]

polish(input)