How hard is LeetCode 2489. Number of Substrings With Fixed Ratio?

LeetCode 2489. Number of Substrings With Fixed Ratio is rated Medium on LeetCode.

What is the time complexity of LeetCode 2489. Number of Substrings With Fixed Ratio?

The Python solution on this page runs in O(n).

What is the space complexity of LeetCode 2489. Number of Substrings With Fixed Ratio?

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

What topics does LeetCode 2489. Number of Substrings With Fixed Ratio cover?

LeetCode 2489. Number of Substrings With Fixed Ratio is tagged Hash Table, Math, String and Prefix Sum on LeetCode.

Is LeetCode 2489. Number of Substrings With Fixed Ratio a premium problem?

Yes. LeetCode 2489. Number of Substrings With Fixed Ratio is a LeetCode Premium problem, so the full statement and test cases require a paid LeetCode subscription.

Number of Substrings With Fixed Ratio — LeetCode 2489 Python Solution

MediumLeetCode PremiumHash TableMathStringPrefix Sum

Problem: #2489
Pattern: Prefix Sum
Reading time: 3 min
Source: leetcode.com

The problem

You are given a binary string s, and two integers num1 and num2. num1 and num2 are coprime numbers.

Example

Input: s = "0110011", num1 = 1, num2 = 2
Output: 4
Explanation: There exist 4 non-empty ratio substrings.

Python solution

Python

class Solution:
    def fixedRatio(self, s: str, num1: int, num2: int) -> int:
        n0 = n1 = 0
        ans = 0
        cnt = Counter({0: 1})
        for c in s:
            n0 += c == '0'
            n1 += c == '1'
            x = n1 * num1 - n0 * num2
            ans += cnt[x]
            cnt[x] += 1
        return ans

Complexity

Measure	Complexity
Time	O(n)
Space	O(n) auxiliary

Pattern: Prefix Sum

Precompute running totals once so any range query becomes a single subtraction. LeetCode 2489. Number of Substrings With Fixed Ratio is filed here because LeetCode tags it Prefix Sum, which is the vocabulary this hub collects.

The prefix sum guide has the Python template for the pattern and the 157 LeetCode problems that use it.

Frequently asked questions

How hard is LeetCode 2489. Number of Substrings With Fixed Ratio?: LeetCode 2489. Number of Substrings With Fixed Ratio is rated Medium on LeetCode.
What is the time complexity of LeetCode 2489. Number of Substrings With Fixed Ratio?: The Python solution on this page runs in O(n).
What is the space complexity of LeetCode 2489. Number of Substrings With Fixed Ratio?: The Python solution on this page uses O(n) auxiliary space.
What topics does LeetCode 2489. Number of Substrings With Fixed Ratio cover?: LeetCode 2489. Number of Substrings With Fixed Ratio is tagged Hash Table, Math, String and Prefix Sum on LeetCode.
Is LeetCode 2489. Number of Substrings With Fixed Ratio a premium problem?: Yes. LeetCode 2489. Number of Substrings With Fixed Ratio is a LeetCode Premium problem, so the full statement and test cases require a paid LeetCode subscription.