38. Count and Say

最新推荐文章于 2020-04-03 01:01:57 发布

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本文介绍了一种生成计数与描述序列的方法，通过递推的方式生成序列中的每一项，并详细解释了如何从上一项得到下一项的具体步骤。示例展示了输入为1和4时的输出结果。

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

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 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”

class Solution {
public:
    string countAndSay(int n) {
        vector<string> dp(n,"");
        char str[2];
        str[1] = '\0';

        if(n<=0){
            return NULL;
        }

        dp[0] = "1";
        for(int i = 1;i < n; ++i){
            const char * p = dp[i-1].c_str();
            int len = dp[i-1].size();
            const char * start = p;
            int j;
            for(j = 0;j<len;j++){
                if(*(start) == p[j]){
                    continue;
                }else{
                    str[0] = *start;
                    dp[i] = dp[i] + to_string(p+j-start) + string(str);
                    start = p + j;           
                }
            }
            str[0] = *start;
            dp[i] = dp[i] + to_string(p+j-start) + string(str);
        }

        return dp[n-1];
    }
};