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