LeetCode "498. Diagonal Traverse"

class Solution(object):
def findDiagonalOrder(self, matrix):
r = []
n = len(matrix)
if n == 0: return r
m = len(matrix[0])

ni = n + m - 1
for i in range(ni):
rr = []
x = min(n - 1, i)
y = max(i - n + 1, 0)
while x >= 0 and y < m:
rr += [matrix[x][y]]
x = x - 1
y = y + 1
if i % 2 == 1:
rr.reverse()
r = r + rr

return r