【leetcode】第38题 Count and Say 题目+解析+代码

【题目】

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"

【解析】

n=2是把n=1读出来，n=3是把n=2读出来，n=4是把n=3读出来……m个x读作mx。

这里用的StringBuilder，原因见上一篇文章。

【代码】

class Solution {
    public String countAndSay(int n) {
        String str="1";
        for(int i=1;i<n;i++)
        {
           str=countAnd(str);
        }
        return str;
    }
    public String countAnd(String str){
        char m=str.charAt(0);
        int count=0;
        StringBuilder st=new StringBuilder("");
        for(int i=0;i<str.length();i++)
        {
            if(str.charAt(i)==m){
                count++;
            }
            else{
                st.append(count).append(m);
                m=str.charAt(i);
                count=1;
            }
        }
        st.append(count).append(m);
        return st.toString();
    }
}