LeetCode题解：Count and Say

计数并描述序列生成

最新推荐文章于 2025-09-08 16:25:29 发布

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本文介绍了一种生成计数并描述序列的方法，通过扫描前一个生成的字符串，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。文章提供了一个高效的实现方案，利用历史记录加速计算过程。

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 n^th sequence.

Note: The sequence of integers will be represented as a string.

思路：

依次扫描之前生成的字符串即可。用一个vector来保存之前的结果，便于之后计算。

题解：

class Solution {
public:
    // save previous history for accelerated calculation
    vector<string> history;
    
    Solution()
    {
        history.emplace_back(string("1"));
    }
    
    string generate_seq(const string& in)
    {
        string generated;
        char scratch[64];
        char current = in[0];
        int count = 0;
        
        auto commit = [&]()
        {
            sprintf(scratch, "%d%c", count, current);
            generated += scratch;
        };
        
        for(auto ch : in)
            if (ch != current)
            {
                // we met another character
                commit();

                count = 1;
                current = ch;
            }
            else
                ++count;
        
        commit();   // commit the last continuous char accumulation
        return generated;
    }
    
    string countAndSay(int n) 
    {
        while(n-1 >= history.size())
            // exploit the r-value reference
            history.emplace_back(generate_seq(history.back()));
            
        return history[n-1];
    }
};