Count and Say

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 n^th sequence.

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

Idea: use a hashmap to store the previous computed results

public class Solution {
    public String countAndSay(int n) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if (n == 0) return "";
        String str = "1";
        HashMap map = new HashMap();
        for (int i = 1; i < n; ++i) {
            int start = 0;
            StringBuffer newstr = new StringBuffer();
            for (int j = 0; j < str.length(); ++j) {
                if (j != str.length() - 1 && str.charAt(j) == str.charAt(j+1)) {
                    continue;
                }
                String stored = map.get(str.substring(start, j+1));
                if (stored == null) {
                    String strappend = String.valueOf(j - start + 1) + str.substring(start, start + 1);
                    map.put(str.substring(start, j+1), strappend);
                    newstr.append(strappend);
                }
                else {
                    newstr.append(stored);
                }
                start = j + 1;
            }
            str = newstr.toString();
        }
        return str;
    }
}