415. 字符串相加

func addStrings(num1 string, num2 string) string {
    sum := ""
    i := len(num1)-1
    j := len(num2)-1
    add := 0

    for i>=0 || j>=0 || add>0 {
        x := 0
        if i>=0 {
            x = int(num1[i]-'0')
        }
        y := 0
        if j>=0 {
            y = int(num2[j]-'0')
        }
        
        res := x+y+add
        sum = strconv.Itoa(res%10) + sum
        add = res/10

        i--
        j--
    }
    return sum
}