LeetCode-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 n, generate the nth sequence.

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

一道facebook的面试题，注意化繁为简。

public class Solution {
    public String countAndSay(int n) {
        if(n<1){
            return null;
        }
        String str="1";
        for(int i=2; i<=n; i++){
            str=countAndSay(str);
        }
        return str;
    }
    
    public String countAndSay(String str){
        StringBuilder builder=new StringBuilder();
        int count=1;
        for (int i=1; i<str.length(); i++){
            if(str.charAt(i) == str.charAt(i-1)){
                count++;
            }
            else{
                builder.append(count);
                builder.append(str.charAt(i-1));
                count=1;
            }
        }
        builder.append(count);
        builder.append(str.charAt(str.length()-1));
        return builder.toString();
    }
}