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.

思路： 一个数字A连续出现N次，就读成”NA”，

实现如下：

class Solution {
public:
    //int to string
    string tostr(int c){
        stringstream ss;
        ss<<c;
        return ss.str();
    }
    //char to string
    string ctostr(char c){
        stringstream ss;
        ss<<c;
        return ss.str();
    }
    string countAndSay(int n) {
        string s="1";
        for(int i=2;i<=n;i++){
            char last='A';//不是'0'-'9'的字符即可
            int cnt=0;//记录last出现的次数
            string r="";
            for(int j=0;j<s.length();j++){
                if(last=='A'){
                    last=s[j];
                    cnt=1;
                }else if(last==s[j]){//重复,+1
                    cnt++;
                }else{
                    r+=tostr(cnt);
                    r+=ctostr(last);

                    cnt=1;
                    last=s[j];
                }
            }
            r+=tostr(cnt);//important
            r+=ctostr(last);//important
            s=r;
        }
        return s;
    }
};