LeetCode 38. Count and Say

Description

https://leetcode.com/problems/count-and-say/

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

Explanation

The base case is when n = 1. For other case, we call countAndSay(n – 1) to get the previous string and build the count and say string by grouping and counting the consecutive numbers.

Python Solution

class Solution:
    def countAndSay(self, n: int) -> str:
        if n == 1:
            return "1"
        else:
            prev_sequence = self.countAndSay(n - 1)
            
            result_sequence = ""
            counter = {}
            for i in range(0, len(prev_sequence)):
                digit = prev_sequence[i]
                                
                if digit not in counter:
                    for key, value in counter.items():
                        result_sequence += str(value) + key                          
                    counter = {}
                    counter[digit] = 1
                                
                else:
                    counter[digit] += 1
            
            for key, value in counter.items():
                result_sequence += str(value) + key              
            
            return result_sequence
  • Time complexity: O(N*M). M is the longest sequence length.
  • Space complexity: O(N).

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