415. 字符串相加

执行用时：8 ms, 在所有 Swift 提交中击败了100.00%的用户
内存消耗：14.1 MB, 在所有 Swift 提交中击败了53.17%的用户

class Solution {
    func addStrings(_ num1: String, _ num2: String) -> String {
        let zero = String.Element("0").asciiValue!
        let numA = Array(num1)
        let numB = Array(num2)
        var add = 0
        var res = ""
        var m = num1.count-1, n = num2.count-1
        while m >= 0 || n >= 0 || add != 0 {
            var x = m >= 0 ? Int(numA[m].asciiValue! - zero) : 0
            var y = n >= 0 ? Int(numB[n].asciiValue! - zero) : 0
            var cur = x + y + add
            res.append(String(cur % 10))
            add = cur / 10
            m -= 1
            n -= 1
        }
        let result = String(res.reversed())
        return result
    }
}