38. Count and Say

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"

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 递归读取前面的数。注意读取时边界的问题

class Solution {
public:
    string cv(int num) {
        string t = "";
        while(num > 0) {
            t += (char)(num % 10 + '0');
            num /= 10;
        }
        reverse(t.begin(), t.end());
        return t;
    }
    string countAndSay(int n) {
        if (n == 1) return "1";
        else {
            string k = countAndSay(n - 1);
            string tmp("");
            int len = k.length();
            if (len == 1) return "11";
            int num = 1;
            
            for (int i = 1; i < k.length(); ++i) {
                if(k[i] == k[i - 1]) {
                    num++;
                    if (i == k.length() - 1) {
                        tmp += cv(num);
                        tmp += k[i - 1];
                    }
                }
                else {
                    tmp += cv(num);
                    tmp += k[i - 1];
                    num = 1;
                }
            }
            if (num == 1) {
                tmp += cv(num);
                tmp += k[len - 1];
            }
            //cout<<tmp<<"-"<<k<<endl;
            return tmp;
        }
    }
};

 数字转字符串还有 std::to_string（int n）这东西