## sorting
* **inversions** in a sequence are pair of elements that are out of order with respect to the ordering relation. a sorting algorithm is a sequence of operations that reduces inversions to zero.
* a **topological sort** of a directed graph is a way of ordering the list of nodes such that if `(a, b)` is an edge of the graph, then `a` appears before `b`. this type of sorting does not work if a graph has cycles or is not directed.
* because of their efficiencies, you usually want to use either merge sort or quick sort (`O(N log (N)`).
* other type of sorting algorithms can be seen below and in this directory's source code:
---
### merge sort
```python
def ms(array):
if len(array) < 2:
return array
mid = len(array) // 2
left = array[:mid]
right = array[mid:]
result, i, j = [], 0, 0
while i < len(left) and j < len(right):
if left[i] <= right[j]:
result.append(left[i])
i += 1
else:
result.append(right[j])
j += 1
if left[i:]:
result.extend(left[i:])
if right[j:]:
result.extend(right[j:])
return result
```