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.

Tips：

时间复杂度 O(n2 )，空间复杂度 O(1)

Note：

public class Solution {
    public String countAndSay(int n) {
            StringBuilder curr=new StringBuilder("1");
            StringBuilder prev;
            int count;
            char say;
            for (int i=1;i<n;i++){
                prev=curr;
                curr=new StringBuilder();       
                count=1;
                say=prev.charAt(0);

                for (int j=1,len=prev.length();j<len;j++){
                    if (prev.charAt(j)!=say){
                        curr.append(count).append(say);
                        count=1;
                        say=prev.charAt(j);
                    }
                    else count++;
                }
                curr.append(count).append(say);
            }                       
            return curr.toString();
    }
}