Table of Contents

## Question

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 substrings such that each substring contains exactly **one** unique digit. Then for each substring, say the number of digits, then say the digit. Finally, concatenate every said digit.

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

:

Given a positive integer `n`

, return *the *`n`

^{th}* term of the count-and-say sequence*.

**Example 1:**

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

**Example 2:**

Input:n = 4Output:"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`

## Python Solution

class Solution: def countAndSay(self, n: int) -> str: if n==1: return '1' prev_res = '1' for _ in range(2,n+1): s = '' c = 0 element = prev_res[0] for j in str(prev_res): if j==element: c+=1 else: s=s+str(c)+str(element) element = j c=1 s=s+str(c)+str(element) prev_res = s return s