LeetCode 274: H-Index — Python Solution

Solve LeetCode 274: H-Index in Python with a sorting and threshold scan approach. The key is to make the state invariant explicit, so the implementation and complexity follow naturally.

This guide paraphrases the task and does not reproduce LeetCode’s prompt. Use the official page for the complete statement, examples, constraints, and submission runner.

What the problem is testing

Sort citations descending and find the largest rank whose citation count is at least that rank.

Algorithm

1. Sort citations descending and find the largest rank whose citation count is at least that rank.
2. Maintain this invariant: The first h sorted papers each have at least h citations.
3. Continue until every input item or reachable state has been resolved, then return the accumulated result.

Python solution

from collections import Counter, defaultdict, deque, OrderedDict
import random

class Solution:
    def hIndex(self, citations):
        citations.sort(reverse=True)
        answer = 0
        for rank, count in enumerate(citations, 1):
            if count < rank:
                break
            answer = rank
        return answer

Why this is correct

The proof follows the maintained state: The first h sorted papers each have at least h citations. Each iteration preserves that claim while permanently resolving at least one position, node, interval, or search state. When the loop or recursion ends, every candidate required by the problem has therefore been included or ruled out, so the returned value is correct.

Complexity

O(n log n) time and O(1) auxiliary space aside from sorting. The stated auxiliary space excludes the returned output unless the output is the data structure being built.

Edge cases

The answer can be zero or the number of papers.

Tested reference code

This implementation is included in the site’s downloadable 100-solution Python library. The complete suite compiles every solution and runs a behavioral assertion for every problem before publication.

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