count-and-say

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38. Count and Say (Medium)

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"

 

Constraints:

• 1 <= n <= 30

Related Topics

[String]

Similar Questions

1. Encode and Decode Strings (Medium)
2. String Compression (Medium)

Hints

Hint 1

The following are the terms from n=1 to n=10 of the count-and-say sequence:

 1.     1
 2.     11
 3.     21
 4.     1211
 5.     111221 
 6.     312211
 7.     13112221
 8.     1113213211
 9.     31131211131221
10.     13211311123113112211

Hint 2

To generate the n^th term, just count and say the n-1^th term.