Leetcode #38: Count and Say
In this guide, we solve Leetcode #38 Count and Say in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
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).
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: String
Intuition
We need to scan characters while tracking positions or counts.
A simple state machine keeps the logic precise.
Approach
Iterate through the string once and update the state for each character.
Use a map or array if you need fast lookups.
Steps:
- Iterate through characters.
- Maintain necessary state.
- Build or validate the output.
Example
countAndSay(1) = "1"
countAndSay(2) = RLE of "1" = "11"
countAndSay(3) = RLE of "11" = "21"
countAndSay(4) = RLE of "21" = "1211"
Python Solution
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
The time complexity is , where n is the input parameter representing the term to generate, and m is the maximum length of the string in the sequence. The space complexity is O(1) to O(n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.