[leetcode] 38. Count and Say

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本文介绍了一种特殊的整数序列——计数与描述序列，并提供了一个C++实现方案。该序列从1开始，后续每一项都是对前一项的描述。例如，1被描述为“一个1”，即11；11被描述为“两个1”，即21。文章通过一个具体的类和方法实现了给定任意正整数n时生成第n项序列。

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.

解法一：

这里的难点在于使用ret.push_back('$')（除了数字以外任意符号）来处理boundary，来简化code。

class Solution {
public:
    string countAndSay(int n) {
        if (n<0) return NULL;
        string ret = "1";
        for(int i =1; i <n; i++){
            string tmp;
            ret.push_back('$');
            int t = 1;
            cout << ret.size() << endl;
            for (int j = 1; j<ret.size(); ++j){
                if (ret[j] == ret[j-1])
                    t++;
                else{
                    tmp.push_back(t+'0');
                    tmp.push_back(ret[j-1]);
                    t = 1;
                }
            }
            ret = tmp;
        }
        return ret;
    }
};