Follow up for H-Index: What if the citations array is sorted in ascending order? Could you optimize your algorithm?

Hint:

1. Expected runtime complexity is in O(log n) and the input is sorted.

274. H-Index H指數 的拓展。輸入的數組是有序的，讓我們優化算法。提示(現在題目中沒有提示了)：O(logn)。

顯然使用二分法。

Python:

class Solution(object):
    def hIndex(self, citations):
        """
        :type citations: List[int]
        :rtype: int
        """
        n = len(citations)
        left, right = 0, n - 1
        while left <= right:
            mid = (left + right) / 2
            if citations[mid] >= n - mid:
                right = mid - 1
            else:
                left = mid + 1
        return n - left

C++:

class Solution {
public:
    int hIndex(vector<int>& citations) {
        int len = citations.size(), left = 0, right = len - 1;
        while (left <= right) {
            int mid = 0.5 * (left + right);
            if (citations[mid] == len - mid) return len - mid;
            else if (citations[mid] > len - mid) right = mid - 1;
            else left = mid + 1;
        }
        return len - left;
    }
};　

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