C# 3 line fastest!
public string ConvertToBase7(int num) {         if (num < 0) return "-" + ConvertToBase7(-num);         if (num < 7) return num.ToString();         return Conve
```
public string ConvertToBase7(int num) {
        if (num < 0) return "-" + ConvertToBase7(-num);
        if (num < 7) return num.ToString();
        return ConvertToBase7(num / 7) + (num % 7).ToString();
    }
```
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