Count and Say

Q:
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 n^th sequence.
Note: The sequence of integers will be represented as a string.

题意是
n=1时输出字符串1；
n=2时，数上次字符串中的数值个数，因为上次字符串有1个1，所以输出11；
n=3时，由于上次字符是11，有2个1，所 以输出21；
n=4时，由于上次字符串是21，有1个2和1个1，所以输出1211。
依次类推，写个countAndSay(n)函数返回字符串。
A:

public class Solution {
   public String countAndSay(int n) {
        // recursively
        if (n == 1) {
            return "1";
        } else {
            String lastStr = countAndSay(n - 1);
            StringBuffer buf = new StringBuffer();
            // use two pointer to check
            int slowRunner = 0, fastRunner;
            while (slowRunner < lastStr.length()) {
                fastRunner = slowRunner;
                while (fastRunner < lastStr.length()) {
                    if (lastStr.charAt(fastRunner) == lastStr
                            .charAt(slowRunner)) {
                        fastRunner++;
                    }else{
                        break;
                    }
                }
                // now fastRunner point to the next value that 
             //either != slowRunner, or exceed array boundary
                buf.append( fastRunner-slowRunner);
                buf.append( lastStr.charAt(slowRunner));
                slowRunner = fastRunner;
            }
            return buf.toString();
        }
    }
}

Mistakes:
1: 当 update fastRunner的时候，当值与slowRunner 不相等的时候，我们要break 该while Loop。 ------------  开始，就SB， 没有加上。
2：

Learned:

----------------------第一遍是递归调用--------第二遍是-------------DP 建立--------------

public class Solution {
    public String countAndSay(int n) {
        // assume n >=1
        String lastStr = "1";
        StringBuffer buf;
        for (int i = 2; i <= n; i++) { // DP calculate each layer
            buf = new StringBuffer();
            for (int j = 0; j < lastStr.length(); j++) {
                char curCh = lastStr.charAt(j);
                int count = 1;
                while (j + 1 < lastStr.length() && lastStr.charAt(j + 1) == curCh) {
                    count++;
                    j++;
                }
                buf.append("" + count + curCh);
            }
            lastStr = buf.toString();
        }
        return lastStr;
    }
}

public class Solution {
    public String countAndSay(int n) {
        String res = "1";
        for(int i =0; i<n-1;i++)
            res = helper(res);
        return res;
    }
    private String helper(String str){
        StringBuffer buf = new StringBuffer();
        int  i = 0;
        while(i<str.length()){
            int len=1;
            while(i+1<str.length() && str.charAt(i)==str.charAt(i+1)){
                len++;
                i++;
            }
            buf.append(""+len+str.charAt(i));
            i++;
        }
        return buf.toString();
    }
}