Swift implementation of Hash Map

HashMap.swift

import UIKit

var string = "1337"

func convert(string: String) -> Int? {
    
    var total = 0
    let valueMap = [
        "0" as Character : 0,
        "1" : 1,
        "2" : 2,
        "3" : 3,
        "4" : 4,
        "5" : 5,
        "6" : 6,
        "7" : 7,
        "8" : 8,
        "9" : 9,
    ]
    
    //Two ways to solve this problem
    //1337 = 1000 + 300 + 30 + 7
    //1337 = 1*10^3 + 3*10^2 + 3*10^1 + 7*10^0
    
    for (index, character) in string.enumerated() {
        let exponent = string.count - index - 1
        if let vaule = valueMap[character] {
            let number = Decimal(vaule) * pow(10, exponent)
            total += NSDecimalNumber(decimal: number).intValue
        } else {
            return nil
        }
    }
    return total
}

convert(string: string)