Count and Say

来源：Leetcode

原帖：http://oj.leetcode.com/problems/count-and-say/

题目：
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.

代码：

 class Solution {  
 public:  
   string countAndSay(int n) {  
     string res = "1";  
     for (int i = 2; i <= n; ++i) { // from 1 to n  
       stringstream ss;  
       int start = 0, N = res.size();  
       while (start < N) {  
         int cur = start + 1;  
         while (cur < N && res[cur] == res[start]) {  
           cur++;            
         }  
         ss << (cur - start) << res[start];  
         start = cur;  
       }  
       ss >> res;  
     }  
     return res;  
   }  
 };  
   
 class Solution {  
 public:  
   string countAndSay(int n) {  
     if(n == 0) return "";  
     string s = "1";  
     for (int i = 1; i < n; i++){  
       s = countAndSay(s);  
     }  
     return s;  
   }  
     
   string countAndSay(string s){  
     string str;  
     for(int i = 0; i < s.size(); i++){  
       int count = 1;  
       while(i + 1 < s.size() && s[i+1] == s[i]){  
         count++;   
         i++;  
       }  
       str += to_string(count);  
       str += s[i];  
     }  
     return str;  
   }  
 };