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.

双指针，记录相同的出现的字符的个数 ；

public class Solution {
    public String countAndSay(int n) {
     
       String say = "1";
       for(int i = 1; i < n ; i++){
           say = build(say);
       }
       return say;
    }
    
    public String build(String s){
        int len = s.length();
        int last = 0;
        String temp = "";
        for(int i = 0; i < len; i++){
            if(s.charAt(last) != s.charAt(i)){
                temp = temp + (i-last)+s.charAt(last);
                last = i;
            }
        }
        // 最后i走到length的时候 还是和last index字符一样，所以要把这个加上
        if(last < len){
            temp = temp + (len -last) + s.charAt(last);
        }
        
        return temp;
    }
}