LeetCode: Count and Say 解题报告

Count and Say

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.

SOLUTION 1:

使用递归来做。这个其实是说要解释几次的问题。

以下这里是解释了3次，你还可以继续用同样的方法继续解释下去，四次五次这样子。

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.

直接看代码吧。也没有什么特别难的地儿。

public class Solution {
    public String countAndSay(int n) {
        if (n == 0) {
            return null;
        }
        
        if (n == 1) {
            return "1";
        }
        
        String s = countAndSay(n - 1);
        StringBuilder sb = new StringBuilder();
        
        int len = s.length();
        int cnt = 0;
        for (int i = 0; i < len; i++) {
            cnt++;
            
            if (i == len - 1 || (i < len - 1 && s.charAt(i) != s.charAt(i + 1))) {
                sb.append(cnt);
                sb.append(s.charAt(i));
                cnt = 0;
            }
        }
        
        return sb.toString();
    }
}

GITHUB:

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/string/CountAndSay.java