38. Count and Say

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 nth sequence.

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

Thought

這題雖然是 Easy 但讀了好久才懂題目的意思。數列如同題目所說，就是 1, 11, 21, 1211, 111221, ... 一直往後長下去直到長度 n 為止的字串為何。例如 n=5，數列的演算過程為 1 (n=1), 11, 21 (n=3), 1211, 111221 (n=5) ，所以回傳 111221 為答案。解法為去統計重複出現的數字次數。

Solution

public class Solution {
    public String countAndSay(int n) {
        if (n <= 0) return null;

        String result = "1";    // the 1st element in the sequence
        int i = 1;

        while (i < n) {
            StringBuilder sb = new StringBuilder();
            int count = 1;
            // compare with the previous string to count the numbers
            // if result = "11", j = 0 stands for the count, j = 1 for the number
            for (int j = 1; j < result.length(); j++) {
                if (result.charAt(j) == result.charAt(j - 1)) {
                    count++;
                } else {
                    // if not the same chars such as "21", store the count and the number,
                    // the reset the count for the next number
                    sb.append(count);
                    sb.append(result.charAt(j - 1));
                    count = 1;
                }
            }

            // to sum up the count of the last number
            sb.append(count);
            sb.append(result.charAt(result.length() - 1));
            result = sb.toString();
            i++;
        }

        return result;
    }
}