274. H-Index

问题描述

Given an array of citations (each citation is a non-negative integer) of a researcher, write a function to compute the researcher's h-index.
According to the definition of h-index on Wikipedia: "A scientist has index h if h of his/her N papers have at least h citations each, and the other N − h papers have no more than h citations each."
For example, given citations = [3, 0, 6, 1, 5], which means the researcher has 5 papers in total and each of them had received 3, 0, 6, 1, 5 citations respectively. Since the researcher has 3 papers with at least 3 citations each and the remaining two with no more than 3 citations each, his h-index is 3.
Note: If there are several possible values for h, the maximum one is taken as the h-index.

问题分析

简单做法：将数组排序后，遍历数组找到citation>=n-i的n-i即为h，算法复杂度即排序的复杂度O(nlgn)；
高效做法：设置一个记录数组count，遍历一遍citations记录count，由count得到结果，算法复杂度O(n)。

AC代码

#encoding=utf-8
class Solution(object):
    def hIndex(self, citations):
        """
        :type citations: List[int]
        :rtype: int
        """
        # 注释掉的是朴素做法
        # n = len(citations)
        # if n == 0:
        #     return 0
        # citations.sort()
        # for i in range(n):
        #     paper = n-i
        #     if citations[i] >= paper:
        #         return paper
        # return 0

        n = len(citations)
        if n == 0:
            return 0
        count = [0 for i in range(n+1)]
        for ci in citations:
            if ci > n:
                count[n] += 1
            else:
                count[ci] += 1
        accumu = 0
        for i in range(n, -1, -1):
            accumu += count[i]
            if accumu >= i:
                return i
        return 0