The count-and-say sequence is the sequence of integers beginning as follows:
1, 11, 21, 1211, 111221, ...

1is read off as"one 1"or11.
11is read off as"two 1s"or21.
21is read off as"one 2, thenone 1"or1211.

Given an integer n, generate the n^th sequence.

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

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

思路：模拟题，一遍一遍推即可。

public class Solution {    public String countAndSay(int n) {        if (n <= 0)            return null;        StringBuilder sb = new StringBuilder();        sb.append("1");        for (int i = 0; i < n - 1; i++) {            StringBuilder nsb = new StringBuilder();            for (int j = 0; j < sb.length();) {                char cur = sb.charAt(j);                int cnt = 0;                while (j<sb.length()&&sb.charAt(j) == cur) {                    j++;                    cnt++;                }                nsb.append((char)(cnt + ‘0‘));                nsb.append(cur);            }            sb = nsb;        }        return sb.toString();    }    public static void main(String[] args) {        System.out.println(new Solution().countAndSay(1));        System.out.println(new Solution().countAndSay(2));        System.out.println(new Solution().countAndSay(3));        System.out.println(new Solution().countAndSay(4));        System.out.println(new Solution().countAndSay(5));        System.out.println(new Solution().countAndSay(6));    }}