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.

此算法在leetcde上实际执行时间为3ms。

class Solution {
public:
    string countAndSay(int n) {
        if (n < 1) return "";

        string result("1");
        while (--n) {
                stringstream stream;
                char last = result[0];
                int count = 1;
                for (int i=1; i<result.size(); i++) {
                        if (result[i] == last)
                                count++;
                        else {
                                stream << count << last;
                                last = result[i];
                                count = 1;
                        }
                }

                stream << count << last;
                result = stream.str();
        }
        return result;
    }
};

Count and Say -- leetcode