【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();
}
}