[LeetCode] Count and Say

Posted 2020-10-09 immjc

The count-and-say sequence is the sequence of integers with the first five terms as following:

1.     1
2.     11
3.     21
4.     1211
5.     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 term of the count-and-say sequence.

Note: Each term of the sequence of integers will be represented as a string.

Example 1:

Input: 1
Output: "1"

Example 2:

Input: 4
Output: "1211"

1读作1个1，记为11

11读作2个1，记为21

21读作1个2，1个1，记为1211

1211读作1个1，1个2，2个1，记为111221

。。。

所以后一个数为前一个数中顺序元素个数的组合。

class Solution {
public:
    string countAndSay(int n) {
        if (n == 0)
            return "";
        string res = "1";
        while (--n) {
            string cur = "";
            for (int i = 0; i != res.size(); i++) {
                int cnt = 1;
                while ((i + 1 < res.size()) && (res[i] == res[i + 1])) {
                    cnt++;
                    i++;
                }
                cur += to_string(cnt) + res[i];
            }
            res = cur;
        }
        return res;
    }
};
// 3 ms