LeetCode——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 nth sequence.

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

原题链接：https://oj.leetcode.com/problems/count-and-say/

可以看出，后一个数字是前个数字的读法，21读作1个2，先写下12，1个1，再写下11，连起来1211.

public static String countAndSay(int n) {
if(n <= 0)
return null;
String str = "1";
int count = 1;
for(int i=0;i<n-1;i++){
StringBuilder builder = new StringBuilder();
for(int j=0;j<str.length();j++){
if(j<str.length()-1 && str.charAt(j)==str.charAt(j+1))
count++;
else{
builder.append(count + "" + str.charAt(j));
count = 1;
}
}
str = builder.toString();
}
return str;
}