38. Count and Say

The count-and-say sequence is the sequence of integers with the first
five terms as following:

1. 1
2. 11
3. 21
4. 1211
5. 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 term of the count-and-say sequence.

Note: Each term of the sequence of integers will be represented as a string.

Example 1:

Input: 1
Output: “1”
Example 2:

Input: 4
Output: “1211”

class Solution {public:    string countAndSay(int n) {        vector<string> dp(n,"");        char str[2];        str[1] = '\0';        if(n<=0){            return NULL;        }        dp[0] = "1";        for(int i = 1;i < n; ++i){            const char * p = dp[i-1].c_str();            int len = dp[i-1].size();            const char * start = p;            int j;            for(j = 0;j<len;j++){                if(*(start) == p[j]){                    continue;                }else{                    str[0] = *start;                    dp[i] = dp[i] + to_string(p+j-start) + string(str);                    start = p + j;                           }            }            str[0] = *start;            dp[i] = dp[i] + to_string(p+j-start) + string(str);        }        return dp[n-1];    }};