Leetcode discuss: 273. Integer to English Words

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First practice - C# - Check list - Take more than one hour

Dec. 30, 2020
Introduction
I came cross the algorithm in Leetcode premium Facebook mock onsite, last algorithm in 4. I spent over 35 minutes but I could not finish it.

Check list
I worked on coding using Visual studio, make sure that the code works for "123", "123456", "1234567890".

Here are the check list to record my learning:

1. Design digts, secondDigit, teens array, every thing should start from index 0 to 9 to match digit 0 to 9, easy to call.
2. More on item 1, I added "Zero" into digits, secondDigit, missing Eighteen in teens array;
3. Failed test case 0, add code to return "Zero";
4. Work on space to separate word; Call C# string.TrimEnd, assign to a variable before return;
5. Failed edge case "millon thousand", so I added the array called zeroChecking;
6. I tried to make the code easy to write, so I design a while loop using repeat, for million three digits, thousand three digits, rightmost three digits.
7. The integer is converted to a C# List, so it is always to work on first char in the list, since List.RemoveAt is called after each visit.
8. Logic of length > 8 meaning length at least 9, which should not be length >= 8.
9. Ideas to improve - I need to think about ideas to code in less than 20 minutes.

public class Solution {
    public string NumberToWords(int num) {
        var chars = num.ToString().ToCharArray();
            var expr = new StringBuilder();
            var digits = new string[] {"Zero", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine" };
            var secondDigit = new string[] {"Zero","Ten","Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety" };
            var teens = new string[] { "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen","Nineteen" };


            var length = chars.Length;
        
            if(num == 0)
                return "Zero";
        
            var list = new List<char>(chars);

            if (length > 9)
            {
                var digit = list[0] - '0';
                if (digit >= 1)
                {
                    expr.Append(digits[digit] + " Billion ");
                }

                list.RemoveAt(0);
            }

            int repeat = 0;
            var zeroChecking = new int[3];
        
            while (repeat < 3)
            {
                if ((repeat == 0 && length > 8) ||
                   (repeat == 1 && length > 5) ||
                   (repeat == 2 && length > 2)
                   )
                {
                    var digit = list[0] - '0';
                    if (digit >= 1)
                    {
                        expr.Append(digits[digit] + " Hundred ");
                        zeroChecking[repeat] = 1; 
                    }

                    list.RemoveAt(0);
                }

                var isTeen = false;
                if ((repeat == 0 && length > 7) ||
                   (repeat == 1 && length > 4) ||
                   (repeat == 2 && length > 1))
                {
                    var digit = list[0] - '0';
                    if (digit > 1)
                    {
                        expr.Append(secondDigit[digit] + " ");
                        zeroChecking[repeat] = 1; 
                    }
                    else if (digit == 1)
                    {
                        // expr.Append(secondDigit[digit]);
                        isTeen = true;
                        zeroChecking[repeat] = 1; 
                    }

                    list.RemoveAt(0);
                }

                if ((repeat == 0 && length > 6) ||
                   (repeat == 1 && length > 3) ||
                   (repeat == 2 && length >= 0))
                {
                    var digit = list[0] - '0';
                    if(digit > 0)
                    {
                        zeroChecking[repeat] = 1;     
                    }
                    
                    if (isTeen)
                    {
                        expr.Append(teens[digit] + " ");
                    }
                    else if (digit > 0)
                    {
                        expr.Append(digits[digit] + " ");
                    }

                    list.RemoveAt(0);
                }

                if (repeat == 0 && length > 6)
                {
                    if(zeroChecking[repeat] > 0)
                        expr.Append("Million ");
                }
                else if (repeat == 1 && length > 3)
                {
                    if(zeroChecking[repeat] > 0)
                        expr.Append("Thousand ");
                }

                repeat++;
            }

            var trimed = expr.ToString().TrimEnd();
            return trimed;
        }
}