Count and Say

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

The count-and-say sequence is the sequence of integers with the first five terms as following:

1. 1

2. 11

3. 21

4. 1211

5. 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 term of the count-and-say sequence.

Note: Each term of the sequence of integers will be represented as a string.

Example 1:

Input: 1

Output: "1"

Example 2:

Input: 4

Output: "1211"

Thoughts

题目意思是把上个生成的string 一个个数字数, 然后把count和对应的digit连起来. 比如1211, 读作1个1,1个2,2个1, 因此新的是111221. 所以我们对每个数字计数即可.

Code

class Solution {
    public String countAndSay(int n) {
        if (n == 0) {
            return "0";
        }
        String res = "1";
        for (int i = 2; i <= n; i++) {
            int count = 0;
            StringBuilder sb = new StringBuilder();
            char c = '\0';
            for (int j = 0; j < res.length(); j++) {
                c = res.charAt(j);
                if (j > 0 && c != res.charAt(j - 1)) {
                    sb.append(count + "" + res.charAt(j - 1));
                    count = 0;
                }
                count++;
            }
            sb.append(count + "" + c);
            res = sb.toString();
            System.out.println(res);
        }
        return res;
    }
}

Analysis

时间复杂度O(N).