citations is sorted in non-decreasing order; citations[i] is how often paper i was cited. The
h-index is the largest h such that at least h papers have at least h citations each. Return it.
Examples
Input: citations = [0, 1, 3, 5, 6]
Output: 3
Explanation: three papers (3, 5, 6) have at least 3 citations; four papers do not have at least 4.
Input: citations = [1, 2, 100]
Output: 2
Input: citations = [0, 0]
Output: 0
Constraints
0 <= len(citations) <= 10**6,0 <= citations[i] <= 10**9- Required time:
O(log n).
Goals
- Express the h-index condition in terms of an index into the sorted list
- Return the count derived from the boundary, not the boundary itself