LeetCode 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, generate the n^th term of the count-and-say sequence.

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

Example 1:

Input: 1
Output: "1"

 

Example 2:

Input: 4
Output: "1211"

题意：

当n=1,str="1"
当n=2,因为当n=1时，str="1"，里面有一个“1”，所以此时的str="11"
当n=3,因为当n=2时，str="11"，里面有两个“1”，所以此时的str="21"
当n=4,因为当n=3时，str="21"，里面有一个"2"，一个“1”，所以此时的str="1211"
当n=5,因为当n=4时，str="1211"，里面是：先一个"1"，再一个"2"，最后两个“1”，所以此时的str="111221"
即：第i+1个字符串是第i个字符串的读法。

思路：直接模拟解决，代码如下：

String s = "1";
        int count = 1, j;
        StringBuilder str = new StringBuilder();
        for(int i = 2; i <= n; i++){
            for(j = 0; j < s.length(); j++){
                while(j + 1 < s.length() && s.charAt(j) == s.charAt(j + 1)){
                    j++;
                    count++;
                }
                str.append(Integer.toString(count) + s.charAt(j));
                count = 1;
             }
            s = str.toString();
            str.delete(0, str.length());
        }
         return s;
    }