leetcode 038 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.

#include <iostream>
#include <cstdio>
#include <string>
#include <cstring>
using namespace std;

class Solution {
public:
string countAndSay(int n) {

if(n == 0) return NULL;

if(n==1) {
return "1";
}

string ret = countAndSay(n-1);
string ans = "";
int len = ret.length();

char pre = ret[0], count = '1';

for(int i=1; i < len; i++) {
if(ret[i] == pre) {
count++;
} else {
ans=ans+count+pre;
count = '1';
pre = ret[i];
}
}

ans = ans+count+pre;

return ans;
}
}