LeetCode - Count and Say | Darren's Blog

The count-and-say sequence is the sequence of integers beginning as follows: 1, 11, 21, 1211, 111221, ...
1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.
Given an integer n, generate the n-th sequence.
Note: The sequence of integers will be represented as a string.

public String countAndSay(int n) {

        if (n < 0)

            return "";

        // Go n-1 steps starting from the first integer "1"

        String result = "1";

        for (int i = 1; i < n; i++) {

            // StringBuilder: a mutable sequence of characters similar to StringBuffer

            // but with no guarantee of synchronization; recommended for most cases

            StringBuilder builder = new StringBuilder();

            int count = 1;      // The number of appearances of the same digit

            for (int j = 1; j < result.length(); j++) {

                if (result.charAt(j) == result.charAt(j-1)) {   // Same as the preceding digit

                    count++;

                } else {        // A different digit

                    builder.append(count);      // Append the count of appearances of the character

                    builder.append(result.charAt(j-1));     // Append the character itself

                    count = 1;      // Reset count

                }

            }

            // The last digit and its count of appearances have not been included in builder

            builder.append(count);

            builder.append(result.charAt(result.length()-1));

            // Get the next integer in the sequence

            result = builder.toString();

        }

        // Out of loop; we arrive at the n-th integer

        return result;

    }

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