What is the time complexity of LeetCode 1180. Count Substrings with Only One Distinct Letter?

The Python solution on this page runs in O(n), where n is the length of the string s.

What is the space complexity of LeetCode 1180. Count Substrings with Only One Distinct Letter?

The Python solution on this page uses O(1) auxiliary space.

Count Substrings with Only One Distinct Letter — LeetCode 1180 Python Solution

EasyLeetCode PremiumMathString

Problem: #1180
Pattern: Math and Number Theory
Reading time: 2 min
Source: leetcode.com

The problem

Given a string s, return the number of substrings that have only one distinct letter.

Example

Input: s = "aaaba"
Output: 8
Explanation: The substrings with one distinct letter are "aaa", "aa", "a", "b".

Python solution

Python

class Solution:
    def countLetters(self, s: str) -> int:
        n = len(s)
        i = ans = 0
        while i < n:
            j = i
            while j < n and s[j] == s[i]:
                j += 1
            ans += (1 + j - i) * (j - i) // 2
            i = j
        return ans

Complexity

Measure	Complexity
Time	O(n), where n is the length of the string s
Space	O(1) auxiliary

Pattern: Math and Number Theory

Find the closed form, the invariant, or the modular identity — and skip the loop entirely. LeetCode 1180. Count Substrings with Only One Distinct Letter is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Math.

The math and number theory guide has the Python template for the pattern and the 485 LeetCode problems that use it.

Frequently asked questions

How hard is LeetCode 1180. Count Substrings with Only One Distinct Letter?: LeetCode 1180. Count Substrings with Only One Distinct Letter is rated Easy on LeetCode.
What is the time complexity of LeetCode 1180. Count Substrings with Only One Distinct Letter?: The Python solution on this page runs in O(n), where n is the length of the string s.
What is the space complexity of LeetCode 1180. Count Substrings with Only One Distinct Letter?: The Python solution on this page uses O(1) auxiliary space.
What topics does LeetCode 1180. Count Substrings with Only One Distinct Letter cover?: LeetCode 1180. Count Substrings with Only One Distinct Letter is tagged Math and String on LeetCode.
Is LeetCode 1180. Count Substrings with Only One Distinct Letter a premium problem?: Yes. LeetCode 1180. Count Substrings with Only One Distinct Letter is a LeetCode Premium problem, so the full statement and test cases require a paid LeetCode subscription.