leetcode : Count and Say [基本功]

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 nth sequence.

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

tag ：字符串的处理。 

注意： 不要忘记最后一个字符的处理

public class Solution {
    
    /*
    1
    11
    21
    1211
    111221
    312211
    13112221
    */
    public String countAndSay(int n) {
       
       if(n <= 0) {
           return "";
       }
       String res = "1";
       int start = 1;
       
       while(start < n) {
           StringBuilder sb = new StringBuilder();
           int count = 1;
           for(int i = 1; i < res.length(); i++) {
               if(res.charAt(i) == res.charAt(i - 1)) {
                   count++;
               } else {
                   sb.append(count);
                   sb.append(res.charAt(i - 1));
                   count = 1;
               }
           }
           sb.append(count);
           sb.append(res.charAt(res.length() - 1));
           res = sb.toString();
           start++;
       }
       return res;
       
    }
}