leetcode Count and Say 2.13 难度系数2

yundomo

于 2014-01-27 12:38:47 发布

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本文介绍了一种特殊的整数序列——计数与描述序列，并提供了一个Java实现的方法来生成该序列的第n项。通过解析序列的形成规则，文章详细展示了如何用编程方式递归地构建序列。

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Question：

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.

public class Solution {
  public static String countAndSay(int n) {

		if (n == 1) {
			return "1";
		}

		String s = "1";
		StringBuffer sb = new StringBuffer();
		int count = 0;
		int round = 0;
		int i;
		while (++round < n) {
			count = 1;
			sb.setLength(0);
			for (i = 1; i < s.length(); i++) {
				if (s.charAt(i) == s.charAt(i - 1)) {
					count++;
				} else {
					sb.append(count);
					sb.append(s.charAt(i-1));
					count = 1;
				}
			}
			sb.append(count);
			sb.append(s.charAt(i-1));
			s = sb.toString();
		}
		return sb.toString();
	}
}