Leetcode 504 | base 7

class Solution {
    func convertToBase7(_ num: Int) -> String {
        
        let flag = num < 0
        var src = abs(num)
        var ret = String()

        repeat {
            ret = "\(src % 7)" + ret
            src /= 7
        } while src != 0
        
        return flag ?  "-" + ret : ret
    }
}