題目描述：

Given an array of citations sorted in ascending order (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

Example:

Input: citations = [0,1,3,5,6]
Output: 3 
Explanation: [0,1,3,5,6] means the researcher has 5 papers in total and each of them had 
             received 0, 1, 3, 5, 6 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, her h-index is 3.
 

Note:

If there are several possible values for h, the maximum one is taken as the h-index.

Follow up:

This is a follow up problem to H-Index, where citations is now guaranteed to be sorted in ascending order. Could you solve it in logarithmic time complexity?

題目大意

計算某個研究人員的影響因子。影響因子的計算方式是有h篇影響力至少為h的論文。影響因子是衡量作者生產力和影響力的方式，判斷了他又多少篇影響力很大的論文。和

解題方法

解釋一下樣例：[0,1,3,5,6]，當h=0時表示至少有0篇影響力為0的論文；當h=1時表示至少有1篇影響力為1的論文；當h=3時表示至少有3篇影響力為3的論文；當h=5時表示至少有5篇影響力為5的論文；當h=6時表示至少有6篇影響力為6的論文.顯然符合要求的是隻要有3篇影響力為3的論文。

在274. H-Index中，是先排序再遍歷做的，這個題已經排好了序，所以比274更簡單，使用二分查詢可以快速求解。

我們求的結果就是求影響力x和不小於該影響力的論文個數的最小值，然後再求這個最小值的最大值。

時間複雜度是O(logN)，空間複雜度是O(N)。

class Solution(object):
    def hIndex(self, citations):
        """
        :type citations: List[int]
        :rtype: int
        """
        N = len(citations)
        l, r = 0, N - 1
        H = 0
        while l <= r:
            mid = l + (r - l) / 2
            H = max(H, min(citations[mid], N - mid))
            if citations[mid] < N - mid:
                l = mid + 1
            else:
                r = mid - 1
        return H

參考資料：

日期

2018 年 10 月 6 日 —— 努力看書