38. 外观数列 Count and Say

The count-and-say sequence is a sequence of digit strings defined by the recursive formula:

countAndSay(1) = "1"
countAndSay(n) is the way you would "say" the digit string from countAndSay(n-1), which is then converted into a different digit string.

To determine how you "say" a digit string, split it into the minimal number of groups so that each group is a contiguous section all of the same character. Then for each group, say the number of characters, then say the character. To convert the saying into a digit string, replace the counts with a number and concatenate every saying.

方法：

递归即可

public String countAndSay(int n) {
        if( n <= 0) return "";
        else if(n == 1) return "1";
        String s = countAndSay(n - 1);
        StringBuilder ans = new StringBuilder();
        StringBuilder same = new StringBuilder();
        char pre = s.charAt(0);
        same.append(pre);

        for (int i = 1; i < s.length(); i++){
            char c = s.charAt(i);
            if (c != pre){
                int count = same.length();
                ans.append(count);
                ans.append(pre);
                pre = c;
                same =new StringBuilder();
            }
            same.append(c);

        }
        if (same.length()!=0){
            int count = same.length();
            ans.append(count);
            ans.append(pre);
        }
        return ans.toString();
    }