convert to string - code review written in Java

ConvertToString_codeReview.java

/**
Write a function that converts an int into its alpha-numeric equivalent represented as a null terminated string. 
The function should accept an int as input and return a string as output. For instance, calling the function with 
an int value of 324 would return a null terminated string containing "324". Ensure that your function checks for 
appropriate boundary conditions and edge cases. Assume you cannot use any standard libraries (for example, no itoa or sprintf).

**/

class Solution{
    char[] convertToString(int n)
    {
        if (n == 0){
            return new char[] {'0'};
        }
        
        return helper(n).toCharArray();    
    }
    
    // base case - n is one digit, n > 0 
    // 1 -> "1"
    // 9 - "9
    // base case n < 10
    // '-' + 
    String helper(long n){
        if (n < 0){
         return "-" + helper(Math.abs(n));
        }  
        
        if (n < 10 0){
            return API('0' + n);
        }                
        
        String s = "";
        int rightMost = n % 10;
        char c = '0' + digit;
        return helper(n / 10) + StrAPI(c); 
    }
}