[leetcode] Problem: Count and Say

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  , generate the  th sequence.

Note: The sequence of integers will be represented as a string.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

public static String countAndSay(int n){ String oldNumber = "1"; for (int i=2; i<=n; i++){ char lastDigit = oldNumber.charAt(0); int count = 1; StringBuilder newNumber = new StringBuilder(); for (int j=1; j<oldNumber.length(); j++){ if (oldNumber.charAt(j) == lastDigit){ count ++; } else { newNumber.append(count); newNumber.append(lastDigit); lastDigit = oldNumber.charAt(j); count = 1; } } newNumber.append(count); newNumber.append(lastDigit); oldNumber = newNumber.toString(); } return oldNumber; }