Count and Say Java

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本文介绍了一种特殊的整数序列——计数并说序列，并提供了一个Java实现方法来生成该序列的第n项。通过分析序列的生成规则，文章详细解释了如何使用StringBuilder来高效地构建序列。

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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.

Have you been asked this question in an interview?

Example:
n = 1, print "1"
n = 2, print "2 1"
n = 3, print "1 2 1 1"
n = 4, print "1 1 1 2 2 1"
n = 5, print "3 1 2 2 1 1"
and so on

Key to solve: the String sequence base on
(count previous number's frequency length + number itself) one by one from beginning
Time Complexity: O(n*length of String)
Space Complexity O(length of String)

public class Solution {
    public String countAndSay(int n) {
        //initial base case
         StringBuilder str=new StringBuilder("1");

        for(int i=1;i<n;i++){
            //temp: store for previous case because current case is base on previous one
            StringBuilder temp=new StringBuilder();
            int head=0;
            int len=str.length();
            for(int j=1;j<len;j++){
                if(str.charAt(j)!=str.charAt(j-1)) {
                    int frequency=j-head;
                    temp.append(frequency).append(str.charAt(head));
                    head=j;
                }
            }
            int frequency2=len-head;
            temp.append(frequency2).append(str.charAt(head));
            str=temp;
        }

        return str.toString();
    }
}