I consider using sort to solve the problem, 4 lines should have same length, the other 2 lines have same length

```
def validSquare(self, p1, p2, p3, p4):
"""
:type p1: List[int]
:type p2: List[int]
:type p3: List[int]
:type p4: List[int]
:rtype: bool
"""
def fl(p1,p2):
return (p1[0]-p2[0])** 2 + (p1[1]-p2[1])** 2
length = []
length.append(fl(p1,p2))
length.append(fl(p1,p3))
length.append(fl(p1,p4))
length.append(fl(p2,p3))
length.append(fl(p2,p4))
length.append(fl(p3,p4))
a = sorted(length)
return len(set(a)) == 2 and len(set(a[:4])) == 1 and len(set(a[4:])) == 1```
```