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.public class Solution {
public String countAndSay(int n) {
if(n<1) return "";
String str = "1";
while(--n>0){
StringBuilder sb = new StringBuilder();
char[] arr = str.toCharArray();
char pre = arr[0];
int count = 1;
for(int i=1;i<arr.length;i++){
if(pre!=arr[i]){
sb.append(count).append(pre);
count = 1;
pre = arr[i];
}else{
count++;
}
}
sb.append(count).append(pre);
str = sb.toString();
}
return str;
}
}