38 Count and Say

Leetcode

The count-and-say sequence is a sequence of digit strings defined by the recursive formula:

countAndSay(1) = "1"
countAndSay(n) is the way you would "say" the digit string from countAndSay(n-1), which is then converted into a different digit string.
To determine how you "say" a digit string, split it into the minimal number of groups so that each group is a contiguous section all of the same character. Then for each group, say the number of characters, then say the character. To convert the saying into a digit string, replace the counts with a number and concatenate every saying.

For example, the saying and conversion for digit string "3322251":

Given a positive integer n, return the nth term of the count-and-say sequence.

Example 1:

Input: n = 1
Output: "1"
Explanation: This is the base case.

Example 2:

Input: n = 4
Output: "1211"
Explanation:
countAndSay(1) = "1"
countAndSay(2) = say "1" = one 1 = "11"
countAndSay(3) = say "11" = two 1's = "21"
countAndSay(4) = say "21" = one 2 + one 1 = "12" + "11" = "1211"

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第一个返回的是 "1" ，后面的可以根据前面的生成，写一个生成函数不断调用它生成下一个。

class Solution {
    public String countAndSay(int n) {
        if(n==1) return "1";
        String pre="1";
        String cur=pre;
        for(int i=2;i<=n;++i){
            cur=next(pre);
            pre=cur;
        }
        return cur;
            

    }
    
    private String next(String s){
        int index=-1;
        StringBuilder sb=new StringBuilder();
        while(index+1<s.length()){
            int i=index+1;
            int count=1;
            char c=s.charAt(i);
            while(i+1<s.length() && c==s.charAt(i+1)){
                i++;
                count++;
            }
            sb.append(count).append(c);
            index=i;
        }
        return sb.toString();
    }
}