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

class Solution {public:    string countAndSay(int n) {        string s=this->itoa(1);//第一个        for(int i=2;i<=n;++i){//从第二个开始            s = nextNum(s);        }        return s;    }private:   string itoa(int i){       char buf[10];       memset(buf,0,sizeof(buf));       sprintf(buf,"%d",i);       string s(buf);       return s;   }   string nextNum(string &s){       string res;       int j=1,count =1;       for(;j<s.size();++j){              if(s[j] == s[j-1]) ++count;              else{                  res.push_back('0'+count);                  res.push_back(s[j-1]);                  count=1;              }       }       res.push_back('0'+count);       res.push_back(s[j-1]);       return res;   }};