How hard is LeetCode 38. Count and Say?

LeetCode 38. Count and Say is rated Medium on LeetCode.

What topics does LeetCode 38. Count and Say cover?

LeetCode 38. Count and Say is tagged String on LeetCode.

Count and Say — LeetCode 38 Python Solution

MediumString

Problem: #38
Pattern: Hash Map
Reading time: 3 min
Source: leetcode.com

The problem

The count-and-say sequence is a sequence of digit strings defined by the recursive formula: countAndSay(1) = "1" countAndSay(n) is the run-length encoding of countAndSay(n - 1). Run-length encoding (RLE) is a string compression method that works by replacing consecutive identical characters (repeated 2 or more times) with the concatenation of the character and the number marking the count of the characters (length of the run).

Example

countAndSay(1) = "1"
countAndSay(2) = RLE of "1" = "11"
countAndSay(3) = RLE of "11" = "21"
countAndSay(4) = RLE of "21" = "1211"

Python solution

Python

class Solution:
    def countAndSay(self, n: int) -> str:
        s = '1'
        for _ in range(n - 1):
            i = 0
            t = []
            while i < len(s):
                j = i
                while j < len(s) and s[j] == s[i]:
                    j += 1
                t.append(str(j - i))
                t.append(str(s[i]))
                i = j
            s = ''.join(t)
        return s

Complexity

Measure	Complexity
Time	O(n \times m), where n is the input parameter representing the term to generate, and m is the maximum length of the string in the sequence
Space	O(1) to O(n) auxiliary

Pattern: Hash Map

Trade memory for time: remember what you have seen so the second pass never happens. LeetCode 38. Count and Say is filed here because the reference solution below belongs to the algorithm family this hub collects, even though its LeetCode tags point elsewhere.

The hash map guide has the Python template for the pattern and the 709 LeetCode problems that use it.

Frequently asked questions

How hard is LeetCode 38. Count and Say?: LeetCode 38. Count and Say is rated Medium on LeetCode.
What topics does LeetCode 38. Count and Say cover?: LeetCode 38. Count and Say is tagged String on LeetCode.