【leetcode】Count and Say

From： https://leetcode.com/problems/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.

class Solution {
public:
string countAndSay(int n) {
n = (n>=0)*n-(n<0)*n;
string ans = "1";
for(int i=1; i<n; i++) ans = find(ans);
return ans;
}

string find(string s) {
int index = s.size()-1, pre = index-1;
string ans;
while(index>=0) {
char cur = s[index];
while(pre>=0 && s[pre] == cur) pre--;
ans = cur + ans;
ans = (char)('0' + index - pre) + ans;
index = pre;
pre = pre-1;
}
return ans;
}
};