Hi, here's your problem today. This problem was recently asked by Amazon:

The h-index is a metric that attempts to measure the productivity and citation impact of the publication of a scholar. The definition of the h-index is if a scholar has at least h of their papers cited h times.

Given a list of publications of the number of citations a scholar has, find their h-index.

Example: Input: [3, 5, 0, 1, 3] Output: 3 Explanation: There are 3 publications with 3 or more citations, hence the h-index is 3.

Here's a starting point:

```
def hIndex(publications):
# Fill this in.
print hIndex([5, 3, 3, 1, 0])
# 3
```