CountandSay.java

/**
 * The count-and-say sequence is the sequence of integers beginning as follows: 1, 11, 21, 1211,
 * 111221, ...
 *
 * <p>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.
 *
 * <p>Given an integer n, generate the nth sequence.
 *
 * <p>Note: The sequence of integers will be represented as a string.
 */
public class CountandSay {
  public String countAndSay(int n) {
    StringBuilder s1 = new StringBuilder("1");
    StringBuilder s2 = new StringBuilder();
    for (int i = 1; i < n; i++) {
      int j = 0;
      int len = s1.length();
      while (j < len) {
        int count = 1;
        char c = s1.charAt(j);
        while (j < len - 1 && s1.charAt(j + 1) == s1.charAt(j)) {
          count++;
          j++;
        }
        s2.append(count + "");
        s2.append(c);
        if (j == len - 1) {
          break;
        }
        j++;
      }
      s1 = s2;
      s2 = new StringBuilder();
    }
    return s1.toString();
  }
}