How hard is LeetCode 2955. Number of Same-End Substrings?

LeetCode 2955. Number of Same-End Substrings is rated Medium on LeetCode.

What is the time complexity of LeetCode 2955. Number of Same-End Substrings?

The Python solution on this page runs in O((n + m) \times |\Sigma|).

What is the space complexity of LeetCode 2955. Number of Same-End Substrings?

The Python solution on this page uses O(n \times |\Sigma|) auxiliary space.

What topics does LeetCode 2955. Number of Same-End Substrings cover?

LeetCode 2955. Number of Same-End Substrings is tagged Array, Hash Table, String, Counting and Prefix Sum on LeetCode.

Is LeetCode 2955. Number of Same-End Substrings a premium problem?

Yes. LeetCode 2955. Number of Same-End Substrings is a LeetCode Premium problem, so the full statement and test cases require a paid LeetCode subscription.

Number of Same-End Substrings — LeetCode 2955 Python Solution

MediumLeetCode PremiumArrayHash TableStringCountingPrefix Sum

Problem: #2955
Pattern: Prefix Sum
Reading time: 3 min
Source: leetcode.com

The problem

You are given a 0-indexed string s, and a 2D array of integers queries, where queries[i] = [li, ri] indicates a substring of s starting from the index li and ending at the index ri (both inclusive), i.e. s[li..ri].

Example

Input: s = "abcaab", queries = [[0,0],[1,4],[2,5],[0,5]]
Output: [1,5,5,10]
Explanation: Here is the same-end substrings of each query:

Python solution

Python

class Solution:
    def sameEndSubstringCount(self, s: str, queries: List[List[int]]) -> List[int]:
        n = len(s)
        cs = set(s)
        cnt = {c: [0] * (n + 1) for c in cs}
        for i, a in enumerate(s, 1):
            for c in cs:
                cnt[c][i] = cnt[c][i - 1]
            cnt[a][i] += 1
        ans = []
        for l, r in queries:
            t = r - l + 1
            for c in cs:
                x = cnt[c][r + 1] - cnt[c][l]
                t += x * (x - 1) // 2
            ans.append(t)
        return ans

Complexity

Measure	Complexity
Time	O((n + m) \times \|\Sigma\|)
Space	O(n \times \|\Sigma\|) auxiliary

Pattern: Prefix Sum

Precompute running totals once so any range query becomes a single subtraction. LeetCode 2955. Number of Same-End Substrings 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 2955. Number of Same-End Substrings?: LeetCode 2955. Number of Same-End Substrings is rated Medium on LeetCode.
What is the time complexity of LeetCode 2955. Number of Same-End Substrings?: The Python solution on this page runs in O((n + m) \times |\Sigma|).
What is the space complexity of LeetCode 2955. Number of Same-End Substrings?: The Python solution on this page uses O(n \times |\Sigma|) auxiliary space.
What topics does LeetCode 2955. Number of Same-End Substrings cover?: LeetCode 2955. Number of Same-End Substrings is tagged Array, Hash Table, String, Counting and Prefix Sum on LeetCode.
Is LeetCode 2955. Number of Same-End Substrings a premium problem?: Yes. LeetCode 2955. Number of Same-End Substrings is a LeetCode Premium problem, so the full statement and test cases require a paid LeetCode subscription.