[leetcode][string] 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.

class Solution {
public:
string countAndSay(int n) {
string res;
if (n < 0) return res;
res = "1";
for (int i = 1; i < n; ++i){
string tmp;
int j = 0;
int cnt = 0;
for (; j < res.size(); ++j){
if (j == 0 || res[j] == res[j - 1]) {

<span style="white-space:pre">				</span>++cnt;

<span style="white-space:pre">				</span>continue;

<span style="white-space:pre">			</span>}
tmp.push_back(cnt + '0');//前一个字符的个数
tmp.push_back(res[j - 1]);//前一个字符
cnt = 1;//！！！当前的字符个数是1个
}
//！！！最后一组
tmp.push_back(cnt + '0');
tmp.push_back(res[j - 1]);
res = tmp;
}
return res;
}
};

注意：在当前字符不同于前一个字符时才可以确定前一组