observation: by definition, an h-index is at most the number of papers.

```
class Solution(object):
def hIndex(self, citations):
"""
:type citations: List[int]
:rtype: int
"""
l = len(citations)
count = [0] * (l + 1)
for c in citations:
count[min(l, c)] += 1
sum = 0
for i in range(l, -1, -1):
sum += count[i]
if sum >= i:
return i
```