An elegant way to walk a matrix along its minor diagonals. Minor diagonal d is made of all elements mat[i][j] such that i + j = d. Minor diagonal d is walked towards row 0 if d is even; if d is odd it is walked towards column 0.

diagonal_matrix_walk.py

from collections import deque
from itertools import chain
def diagonal_walk(mat):
    m, n = len(mat), len(mat[0])
    diagonals = [deque() for _ in range(m + n - 1)]
    for i in range(m):
        for j in range(n):
            if (i + j) % 2 == 0:
                diagonals[i + j].appendleft(mat[i][j])
            else:
                diagonals[i + j].append(mat[i][j])
    return list(chain(*diagonals))