Diagonal Matrix

gistfile1.py

'''
Notes about this problem:
- Try to analyze the formation of the result & a way to construct it
'''

M = [[1, 2, 3],
     [4, 5, 6],
     [7, 8, 9]]

def mat_diagonal(M):
    res = []
    n = len(M)
    for i in range(0, n * 2 - 1):
        data = []
        for j in range(0, n):
            if i - j < 0 or i - j >= n:
                continue
            data.append(M[i - j][j])
        res.append(data)
    return res

assert mat_diagonal(M) == [ [1], [4, 2], [7, 5, 3], [8, 6], [9]]