```
def kthSmallest_124(self, matrix, k):
"""
:type matrix: List[List[int]]
:type k: int
:rtype: int
"""
flatten = lambda x: [y for l in x for y in flatten(l)] if type(x) is list else [x]
matrix = flatten(matrix)
matrix.sort()
return matrix[k-1]
```

I just used the default sorting method. Could be better.