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.

数字转换为string，使用stringstream流，字符直接使用append追加到string后面。注意，对于流，每次用完要清空。

C++代码实现：

#include<iostream>
#include<string>
#include<sstream>
using namespace std;

class Solution
{
public:
    string countAndSay(int n)
    {
        if(n==0)
            return NULL;
        if(n==1)
            return string("1");
        string s="1";
        int i;
        size_t j,k=0;
        for(i=2; i<=n; i++)
        {
            string tmp;
            stringstream ss;
            for(j=1; j<s.length(); j++)
            {
                if(s[k]!=s[j])
                {
                    ss<<j-k;
                    tmp+=ss.str();
                    //注意清空stringstream，不然上次的内容还存在，还有清空操作不是clear()
                    ss.str("");
                    tmp.append(1,s[k]);
                    k=j;
                }
            }
            ss<<j-k;
            tmp+=ss.str();
            ss.str("");
            tmp.append(1,s[k]);
            s=tmp;
            k=0;
        }
        return s;
    }
};
int main()
{
    Solution s;
    for(int i=1; i<=10; i++)
        cout<<s.countAndSay(i)<<endl;
}

运行结果：

 改进版

#include<iostream>
#include<string>
#include<sstream>
using namespace std;

class Solution
{
public:
    string countAndSay(int n)
    {
        if(n==0)
            return NULL;
        if(n==1)
            return string("1");
        string s="1";
        int i;
        size_t j,k=0;
        for(i=2; i<=n; i++)
        {
            string tmp(s);
            j=0;
            int m=tmp.size();
            s.clear();
            while(j<m)
            {
                k=j+1;
                while(k<m&&tmp[j]==tmp[k]) k++;
                int len=k-j;
                s+=to_string(len);
                s.append(1,tmp[j]);
                j=k;
            }
        }
        return s;
    }
};
int main()
{
    Solution s;
    for(int i=1; i<=10; i++)
        cout<<s.countAndSay(i)<<endl;
}