【LeetCode】38 - 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.

Solution：下一个字符串= 当前字符串：∑(字符个数+字符)，例如1211，下一个字符串=count(1)+'1'+count(2)+'2'+count(1)+'1'=111221

class Solution {
public:
    string countAndSay(int n) {
        string ret="1";
        if(n==1)return ret;
        while(--n){
            string next;
            char last=ret[0];
            int i=1,count=1;
            while(i<ret.size())
            {
                while(ret[i]==last&&i<ret.size()){
                    i++;
                    count++;
                }
                if(i<ret.size()&&ret[i]!=last){
                    next+=to_string(count);
                    next+=last;
                    
                    count=1;
                    last=ret[i];
                    i++;
                }
            }
            next+=to_string(count);
            next+=last;
            ret=next;
        }
        return ret;
    }
};