Tags: "leetcode", "python", access_time 2-min read

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Count and Say

Created: November 3, 2018 by [lek-tin]

Last updated: November 3, 2018

The count-and-say sequence is the sequence of integers with the first five terms as following:

  1. 1
  2. 11
  3. 21
  4. 1211
  5. 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 where 1 ≤ n ≤ 30, generate the nth term of the count-and-say sequence.

Note: Each term of the sequence of integers will be represented as a string.

Example 1

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

Example 2

Input: 4
Output: "1211"
Explanation: For n = 3 the term was "21" in which we have two groups "2" and "1", "2" can be read as "12" which means frequency = 1 and value = 2, the same way "1" is read as "11", so the answer is the concatenation of "12" and "11" which is "1211".

Solution 1

class Solution:
    def countAndSay(self, n):
        """
        :type n: int
        :rtype: str
        """
        i = 1
        res = "1"
        while i < n:
            c = res[0]
            temp = ""
            count = 0
            for j in range(len(res) + 1):
                if j != len(res) and res[j] == c:
                    count += 1
                else:
                    # Iteration finished or encountered different number than "c"
                    temp += str(count)
                    temp += c
                    if j != len(res):
                        c = res[j]
                        count = 1
            res = temp
            i += 1
        return res

Solution 2

import time
class Solution:
    def countAndSay(self, n: int) -> str:
        digits = "1"

        if n == 1:
            return digits

        for i in range(n-1):
            start = 0
            count = 1
            N = len(digits)
            newDigits = ""
            while start < N:
                i = start + 1
                while i < N and digits[i] == digits[i-1]:
                    count += 1
                    i += 1
                newDigits += str(count) + str(digits[start])
                count = 1 
                start = i
                i += 1
            digits = newDigits

        return digits