38. Count and Say.md

38. Count and Say

The count-and-say sequence is the sequence of integers with the first five terms as following:

1.     1
2.     11
3.     21
4.     1211
5.     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 where 1 ≤ n ≤ 30, generate the nth term of the count-and-say sequence. You can do so recursively, in other words from the previous member read off the digits, counting the number of digits in groups of the same digit.

Note: Each term of the sequence of integers will be represented as a string.

Example 1:

Input: 1
Output: "1"
Explanation: This is the base case.

Example 2:

Input: 4
Output: "1211"
Explanation: For n = 3 the term was "21" in which we have two groups "2" and "1", "2" can be read as "12" which means frequency = 1 and value = 2, the same way "1" is read as "11", so the answer is the concatenation of "12" and "11" which is "1211".

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Solution

• mine

  can not understand what is mean, but got it when i see the accepted solution.

• the most votes

  Runtime: 1 ms, faster than 89.43%,Memory Usage: 37.3 MB, less than 31.58% of Java online submissions

  public String countAndSay(int n) {
      if(n <= 0) return "-1";
      String result = "1";
  
      for(int i = 1; i < n; i ++) {
          result = build(result);
      }
      return result;
  }
  
  private String build(String result) {
      StringBuilder builder = new StringBuilder();
      int p = 0;
      while(p < result.length()) {
          char val = result.charAt(p);
          int count = 0;
  
          while(p < result.length() && 
            result.charAt(p) == val){
              p ++;
              count ++;
          }
          builder.append(String.valueOf(count));
          builder.append(val);
      }
      return builder.toString();
  }